Space robot reliability is a fatal important problem. Any fault and error may damage spacecraft. This paper suggests a space robot system with two alternate drive system, two alternate control system, redundant freedom, two alternate communicate system. The space robot is a fault-tolerant system. Fault tree is used to analysis space robot. Any one element on the space robot fault doesnpsilat influence robot work. Space robot reliability is raised to a high level through this design. This design scheme is a practice scheme for space robot.