Abstract: Based on slip ratio and terramechanics of the wheels,this paper investigates the problem of excessive sinkage of lunar rover wheel on the loose lunar soil.The sinkage of lunar rover wheel and the wheel-soil interaction forces are modeled as a function of wheel slip ratio.Dynamical model of the lunar rover is established,which incorporates the terramechanics theory and calibrats the parameters of soil by the theory of multi-pass performance of tandem wheels.Then regarding whether the effective traction force supported by the loose soil is larger than the wheel resistance,excessive sinkage of lunar rover wheel is determined.And then the desired value of wheel slip ratio is calculated by this dynamical model,which can avoid excessive sinkage of lunar rover wheel.Finally,considering the nonlinearity and uncertainty of lunar rover systems,the sliding mode controller is designed in terms of wheel slip ratio.Simulation results show that the controller effectively keeps the slip ratio of the driving wheel tracking the desired value and avoids the excessive spin and sinkage of the driving wheel,therefore guarantees the normal operation of the lunar rover on soft soil.