In this paper, a novel nonlinear dynamic model of the solar array drive system (SADS) with spur gear train transmission is proposed considering backlash and friction, which is composed of servo motor, spur gear reducer, solar arrays and friction. The backlash effect of the spur gear reducer is modeled based on an alternate engagement mechanism, while the transmitted torque is defined via the use of a dead zone function. Friction moment in the SADS is described by the hybrid model including static friction, sliding friction and stribeck effect. Based on the above-mentioned model, the influence of backlash and friction on the velocity stability of motor in the SADS is investigated. This paper can provide a guideline for engineers to follow in the control design of solar array drive system.