diff --git a/__tests__/statisticsService.test.ts b/__tests__/statisticsService.test.ts
new file mode 100644
index 0000000..0c129da
--- /dev/null
+++ b/__tests__/statisticsService.test.ts
@@ -0,0 +1,125 @@
+import { calculateActivityStatistics } from "@/lib/statisticsService";
+
+jest.mock("@/lib/models/Activity", () => ({
+  Activity: {
+    findById: jest.fn(),
+  },
+}));
+
+jest.mock("@/lib/models/Option", () => ({
+  Option: {
+    find: jest.fn(),
+  },
+}));
+
+jest.mock("@/lib/models/Vote", () => ({
+  Vote: {
+    find: jest.fn(),
+  },
+}));
+
+const { Activity } = jest.requireMock("@/lib/models/Activity") as {
+  Activity: { findById: jest.Mock };
+};
+const { Option } = jest.requireMock("@/lib/models/Option") as {
+  Option: { find: jest.Mock };
+};
+const { Vote } = jest.requireMock("@/lib/models/Vote") as {
+  Vote: { find: jest.Mock };
+};
+
+const ACTIVITY_ID = "507f1f77bcf86cd799439011";
+const OPTION_A = "507f1f77bcf86cd799439021";
+const OPTION_B = "507f1f77bcf86cd799439022";
+
+function mockActivity(overrides: Record<string, unknown> = {}) {
+  Activity.findById.mockReturnValue({
+    populate: jest.fn().mockResolvedValue({
+      _id: ACTIVITY_ID,
+      name: "測試選舉",
+      type: "election",
+      rule: "choose_one",
+      users: [],
+      eligible_voters_count: 0,
+      open_from: new Date("2026-01-01"),
+      open_to: new Date("2026-12-31"),
+      ...overrides,
+    }),
+  });
+}
+
+function mockOptions() {
+  Option.find.mockResolvedValue([
+    { _id: OPTION_A, candidate: { name: "王小明" } },
+    { _id: OPTION_B, candidate: { name: "陳大文" } },
+  ]);
+}
+
+describe("statistics service", () => {
+  beforeEach(() => {
+    jest.clearAllMocks();
+  });
+
+  it("uses the eligible voter roster as the turnout denominator", async () => {
+    mockActivity({
+      eligible_voters_count: 500,
+      users: ["A", "B", "C", "D", "E"],
+    });
+    mockOptions();
+    Vote.find.mockResolvedValue([
+      { choose_one: OPTION_A },
+      { choose_one: OPTION_A },
+      { choose_one: OPTION_B },
+      { choose_one: OPTION_B },
+      { choose_one: OPTION_B },
+    ]);
+
+    const result = await calculateActivityStatistics(ACTIVITY_ID);
+
+    expect(result.success).toBe(true);
+    // 5 票 / 500 名冊人數 = 1.00%，而不是 5/5 = 100%
+    expect(result.data?.statistics.totalEligibleVoters).toBe(500);
+    expect(result.data?.statistics.totalVotes).toBe(5);
+    expect(result.data?.statistics.turnoutRate).toBe("1.00");
+  });
+
+  it("returns zero turnout when no voter roster has been uploaded", async () => {
+    mockActivity({ eligible_voters_count: 0, users: [] });
+    mockOptions();
+    Vote.find.mockResolvedValue([]);
+
+    const result = await calculateActivityStatistics(ACTIVITY_ID);
+
+    expect(result.success).toBe(true);
+    expect(result.data?.statistics.totalEligibleVoters).toBe(0);
+    expect(result.data?.statistics.turnoutRate).toBe("0");
+  });
+
+  it("counts choose_one votes per option", async () => {
+    mockActivity({ eligible_voters_count: 10 });
+    mockOptions();
+    Vote.find.mockResolvedValue([
+      { choose_one: OPTION_A },
+      { choose_one: OPTION_B },
+      { choose_one: OPTION_B },
+    ]);
+
+    const result = await calculateActivityStatistics(ACTIVITY_ID);
+
+    const stats = result.data?.statistics.optionStats ?? [];
+    expect(stats.find((s) => s.option_id === OPTION_A)?.total).toBe(1);
+    expect(stats.find((s) => s.option_id === OPTION_B)?.total).toBe(2);
+    expect(result.data?.statistics.turnoutRate).toBe("30.00");
+  });
+
+  it("returns 404 when the activity does not exist", async () => {
+    Activity.findById.mockReturnValue({
+      populate: jest.fn().mockResolvedValue(null),
+    });
+
+    const result = await calculateActivityStatistics(ACTIVITY_ID);
+
+    expect(result.success).toBe(false);
+    expect(result.statusCode).toBe(404);
+  });
+});
diff --git a/lib/statisticsService.ts b/lib/statisticsService.ts
index 9f188db..278ab0a 100644
--- a/lib/statisticsService.ts
+++ b/lib/statisticsService.ts
@@ -49,6 +49,7 @@ interface ActivityDocument extends Document {
   type: string;
   rule: string;
   users: string[];
+  eligible_voters_count: number;
   open_from: Date;
   open_to: Date;
 }
@@ -85,7 +86,8 @@ export async function calculateActivityStatistics(
 
     // Calculate basic statistics
     const totalVotes = votes.length;
-    const totalEligibleVoters = activity.users.length;
+    // 分母是名冊人數（上傳選舉人 CSV 時寫入），不是已投票人數
+    const totalEligibleVoters = activity.eligible_voters_count ?? 0;
     const turnoutRate =
       totalEligibleVoters > 0
         ? ((totalVotes / totalEligibleVoters) * 100).toFixed(2)