Today, My Quad Copter ascend while RTL and Land? (Copter V3.4.6)

First of all, it has no relationship to RTL_ALT.

In most time, RTL and Land modes always work fine for me. But today, when I switched mode from Loiter to RTL (throttle was middle up), the drone flied back, but kept ascending. I tried to lower it by lowered the throttle, switched mode to Land, it not worked…

Finally, I succeeded switching to Loiter mode, lowered the drone, and finally, Land mode worked and it landed.

Could you tell me why it happened?

145.BIN (3.1 MB)

Your logs show you have very high vibrations on your copter and the accelerometers are clipping which can cause this behavior.

Try balancing your propellers and adding vibration dampening to your flight controller.

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I checked your log, and found your viberation is too high, very possibly this is the cause of the ascending.
you can reduce the the viberation according to arducopter wiki.

Thanks Justin and Wilson, I will check that.