diff --git a/packages/koishi-utils/src/logger.ts b/packages/koishi-utils/src/logger.ts
index 7c6a836dfd..4b244448f7 100644
--- a/packages/koishi-utils/src/logger.ts
+++ b/packages/koishi-utils/src/logger.ts
@@ -48,7 +48,9 @@ export class Logger {
   private code: number
   private displayName: string
 
-  constructor(private name: string, private showDiff = false) {
+  public stream: NodeJS.WritableStream = process.stderr
+
+  constructor(public name: string, private showDiff = false) {
     if (name in instances) return instances[name]
     let hash = 0
     for (let i = 0; i < name.length; i++) {
@@ -72,7 +74,7 @@ export class Logger {
   private createMethod(name: LogType, prefix: string, minLevel: number) {
     this[name] = (...args: [any, ...any[]]) => {
       if (this.level < minLevel) return
-      process.stderr.write(prefix + this.displayName + this.format(...args) + '\n')
+      this.stream.write(prefix + this.displayName + this.format(...args) + '\n')
     }
   }
 
diff --git a/packages/koishi-utils/tests/logger.spec.ts b/packages/koishi-utils/tests/logger.spec.ts
new file mode 100644
index 0000000000..19c737f49f
--- /dev/null
+++ b/packages/koishi-utils/tests/logger.spec.ts
@@ -0,0 +1,54 @@
+import { install, InstalledClock } from '@sinonjs/fake-timers'
+import { expect } from 'chai'
+import { Logger } from 'koishi-utils'
+import { Writable } from 'stream'
+
+describe('Logger API', () => {
+  let logger: Logger
+  let data: string
+  let clock: InstalledClock
+  const { colors } = Logger.options
+
+  before(() => {
+    Logger.options.colors = false
+    clock = install({ now: Date.now() })
+  })
+
+  after(() => {
+    Logger.options.colors = colors
+    clock.uninstall()
+  })
+
+  it('basic support', () => {
+    logger = new Logger('test').extend('logger', true)
+    expect(logger.name).to.equal('test:logger')
+    logger.stream = new Writable({
+      write(chunk, encoding, callback) {
+        data = chunk.toString()
+        callback()
+      },
+    })
+  })
+
+  it('format error', () => {
+    const error = new Error('message')
+    error.stack = null
+    logger.error(error)
+    expect(data).to.equal('[E] test:logger message\n')
+  })
+
+  it('format object', () => {
+    clock.tick(2)
+    const object = { foo: 'bar' }
+    logger.success(object)
+    expect(data).to.equal("[S] test:logger { foo: 'bar' } +2ms\n")
+  })
+
+  it('custom formatter', () => {
+    clock.tick(1)
+    Logger.formatters.x = () => 'custom'
+    Logger.levels[logger.name] = 2
+    logger.info('%x%%x')
+    expect(data).to.equal('[I] test:logger custom%x +1ms\n')
+  })
+})