Integrating the merits of wheeled and tracked mechanism, a robot’s mobile mechanism may have more adaptability in field environments. A kind of metamorphic mechanism with three configurations was proposed, which configuration transformation was analyzed and metamorphic equations on configuration transformations were put forward. Based on them, a wheel-tracked robot’s mobile mechanism was brought forward. The key component of the structure, locomotion-mode-changing unit, was synthesized. The robot’s prototype was made and the principle test was finished. The result shows that the built prototype can run by wheels or tracks, and transform itself successfully.