For features of unstructured ground environments, considering the moving mechanism of wheel and track in complex terrain environment movement, a new wheel-track transformable wheel, which can changes its travelling mode according to the ground environment, is proposed. Obtain the effect of the transformable wheel mechanism parameters on maximum grounding-length by using sequential quadratic programming (SQP), which provides the theoretical basis for determine the optimization mechanism parameters. Analysis results of mechanism parameters influence on passability show that, with the increase of the diameter-varying ratio, the track mode may result in a more obvious capability in obstacle performance than the wheel mode. Meanwhile, compared with the track mode, the wheel mode is more fast and flexible.