Lumped parameter dynamic modeling is an effective method to understand the fault mechanism of planetary gears. When using lumped parameter model，it is very important to select appropriate variables to represent the response of the system. In view of the problem that the existing methods are not suitable for the response representation of the planetary gear system with fixed sun gear. In this paper，a method is proposed to express the response of planetary gear system by directly superimposing the acceleration along the meshing line. The translation torsion model of the planetary gear is established，and the necessity of selecting three degrees of freedom is explained and the correctness of the model is verified by analyzing the relative displacement of meshing line and meshing force signal and spectrum. Compared with two typical response representation methods，the proposed method has better effect in both normal gear and fault condition，and can effectively extract the meshing frequency and fault frequency. After that，the rationality of the proposed method is further verified by the spectrum analysis of the experimental signals of normal gear and sun gear fault.