Wheeled mobile robots are increasingly being utilized in unknown and dangerous situations such as planetary surface exploration. Based on force analysis of the differential joints and force analysis between the wheels and the ground, this paper established the quasi-static mathematical model of the 6-wheel mobile system of planetary exploration rover with rocker-bogie structure. Considering the constraint conditions, with the method of finding the wheels’friction force solution space feasible region, obstacle-climbing capability of the mobile mechanism was analyzed. Given the same obstacle heights and contact angles of wheel-ground, the single side forward obstacle-climbing of the wheels was simulated respectively, and the results show that the rear wheel has the best obstacle-climbing capability, the middle wheel is the worst, and the front wheel is moderate.