Altitude descend when moving in POSHOLD mode

Hi all:
I fly the pixhawk +Apm 3.2.1,and the frame is 450mm.I meet a prolem when I first fly.
The altitude of copter will descend when I control the copter moving(left/right or forward/back ) in POSHOLD mode .I don’t know how to solve this problem.
someone could help me ?
thanks so much.