After my dji mavic mini crashed a tree, the video recorded becomes unstable. Is it the problem about the gimbal? but I don't see any damage on it. How to repair it? https://youtu.be/SiUTUGHLrhE

Sorry for the troubles with the gimbal, since the drone was crashed, the performance cannot be ensured, it is recommended to send in the drone to our repair center for the evaluation, please email to support@dji.com for further assistance, thanks.

Since you're in Hong Kong, visit them.

https://www.dji.com/service/repair/reserve

I have solved the problem. It's due to the off position of one of the four supports of the gimbal. Thanks for your help.