@modaltb Thank you so much. I followed your instruction and then reboot the quadcopter, then it works. But there is a new issue: When I calibrate the radio, the radio channels for roll, yaw and throttle looks good, but for pitch angle, it shows 'not mapped'. I checked Qgroundcontrol: 'Vehicle Setup->Parameter->Radio Calibration', RC_MAP_ROLL is channel1, the RC_MAP_PITCH is channel2, RC_MAP_THROTTLE is channel3 and RC_MAP_YAW is channel4.
Do you know what's wrong about the pitch radio channel mapping?