Abstract: The mechanical transmission backlash mainly generated by the transmission system gap, the elastic deformation of the transmission chain, the loadeddeformation of mechanical components and other factors, has a great influence on the control precision and stability of servo system. We analyzed the mathematical model of the mechanical transmission backlash, set up a block diagram of servo system considering the transmission backlash. Furthermore, we analyzed the modeling of mechanical transmission backlash and its influence on the servo performance in depth through a typical simulation example. The results of the analysis show that the influence of transmission backlash on servo performance mainly reflects in two aspects: first, it can cause the phase lag and affect the stability of the system; second, it can cause the limit cycle oscillating which produces self-vibration static error.