The obstacle climbing performance was discussed for variable diameter wheel all-terrain-vehicle (ATV) with two kinds of drive mode: rear wheel drive mode and four-wheel drive mode. In RecurdynV7R5 software models of the diameter-variable wheel and a normal tire were built to analyze the climbing obstacles characteristics. On this basis, the vehicle model which would be driven by rear wheel or four wheels mechanical was set up, and the calculation was executed. The results showed that the driving way and the variation of wheel diameter play an important role for obstacle climbing performance. In RecurdynV7R5 software the whole vehicle model with rear wheel drive mode or four wheel drive mode was built, and obstacle climbing performance was simulated in the condition of adjusting the wheel diameter size. Then, the calculation and the simulation were compared. In the end, experimental results verify the reliability between the theoretical calculation and the simulation analysis.