Because two days ago, I upgraded Android studio from version 2 . three . Upgrade 3 to 3 . 3. At the same time, we also upgraded the SDK, including the emulator to 27 . x。 Then the nightmare began. After the simulator starts successfully, the Android studio card cannot be connected, and waiting for target device to come online is displayed. I read the related posts of stack overflow again, but it didn’t work out. In the end, there was no way to do it. They almost wanted to reload the operating system (fortunately, it’s still the same problem after the reload). Before reloading, I took an old SDK from another machine. As a result, I was able to run and finally found the problem with the emulator. Lesson: if you have no problem with IDE, don’t always upgrade. The hairline is rising again.

My system: win7