Abstract: There are a large number of ultra-large slope bridges in the rack railway line， and the settlement of bridge piers is difficult to avoid. It leads to the decrease of the smoothness of the railway line and threaten the safety and smoothness of the train. To solve this problem， based on the theory of vehicle-rack（track）-bridge dynamic interaction and gear dynamics， a coupled dynamic model of mountain vehicle-rack（track） -bridge system is established. The dynamic model considers the nonlinear meshing behavior of gear-rack and nonlinear contact behavior of wheel-rail in detail. The meshing behavior of gear-rack and the dynamic characteristics of vehicles under three different pier settlement modes （single pier settlement， continuous pier settlement and spaced pier settlement） are investigated， and the influence of different pier settlement modes on the vibration of vehicles is compared. The results show that the influence of pier settlement on the rack railway system is mainly reflected in the vertical and longitudinal acceleration of the vehicle， and the main frequency of vibration is 1~2 Hz and 8~9 Hz respectively. The effects of single pier settlement on the basic vibration characteristics of the rack-bridge system are similar to those of double pier settlement. But compared with double pier settlement， the effects of single pier settlement on the longitudinal acceleration of the vehicle are more significant， and the fluctuation of the meshing frequency is also significantly increased. Pier settlement will cause the increase of gear offset， and then lead to the instability of gear-rack meshing. When pier settlement is 6 mm， the problem of meshing apart begins to appear， which seriously threatens the operation safety of vehicles.