To access the complex and unpredictable environment, a reconfigurable wheel-track robot with multiple locomotion modes is developed. It has the merits of both the wheel mechanism and the track mechanism. The robot consists of a control system unit, two symmetric reconfigurable wheels, rollover mechanism and robot body and has three locomotion modes-wheel mode, track mode and rollover mode. In the wheel locomotion mode, it is an omnidirectional two-wheel robot which can achieve high speed. In the track locomotion mode, the robot has a strong obstacle-climbing capability to complex terrain such as gravel road and grass. In the rollover locomotion mode, the robot rolls over to cross vertical obstacle. It can change its locomotion mode by the transformable parallel four-bar linkage according to the complex terrain. Based on the kinematic and mechanical models, optimization design for mechanism was subsequently carried out integrated with numerical simulations. The proposed mechanism can optimize the movement trajectory of transformation mechanism, reduce the performance requirements of motor and improve the motor service efficiency. Experiments show that the robot can efficiently traverse sands and climb vertical railings. It has prominent adaptability to environment and obstacle-surmounting ability. The results prove the design of the mobile mechanism system to be rational.