Abstract: In view of the complex ground environment after disasters or wars, a robot with a variable structure is developed, which can be thrown or fired into the working area. The robot adopting spherical structure can be thrown or fired across some obstacles which are difficult for most existing robots to cross. After going into the working area, the robot can adopt the four-wheel structure with two passive degrees of freedom to realize the moving work. In order to control movement of the robot effectively, a kinematic model is established, in which the geometric constraint and the non lateral slip constraint of the front and rear bodies are considered for the unfolded structure, and the cause of the robot's trajectory error along a curved path is analyzed. The simulation for the robot to follow a sinusoidal trajectory is performed, and the trajectory error and its influencing factors are studied. An experiment is performed to verify the presented model and the simulation results, and good mobility of the robot is demonstrated.