In this article, we comprehensively showed the impact of barrier layer traps, buffer layer traps and surface charge on current collapse and knee voltage change, pointed out that the change in the concentration of surface charge and barrier layer traps have little influence on the 2DEG density in the channel.When the concentration of surface charge changes, the knee voltage drift and strength of current collapse are in close connection with the change of potential energy in quantum well.Buffer layer has stronger local effect than barrier layer, when the concentration of bulk traps change in these layers, knee voltage drift is mainly caused by the change of 2DEG density, but has less change compared with the situation of surface charge.Potential energy changes in the quantum well is an important reason for the change of knee voltage, the strength of current collapse is determined by the size of potential energy and 2DEG density.