Abstract: For wall-climbing robots(WCRs) working at hostile environments at high altitude,the mass of the tethered electric cables or the carried working materials(e.g.,washing liquid,paint) may change with the operating height and the working progress.The variable mass can affect the traction force which determines the locomotion capability and the operational reliability of WCRs.Based on variable mass system mechanics theory,the robotic system is regarded as a variable mass object.For the two basic particle dynamic problems,the dynamic model is given based on variable mass Newton mechanics.Then,the variable mass system dynamic model with nonholonomic constraints are developed for wheeled WCRs.Simulation results show that it is reasonable and necessary to study this kind of WCRs by regarding them as variable mass objects.The new research technique can be utilized to increase the service ability and the adaptability to wall surfaces of WCRs.