Wire-moving robot system is a typical under-actuated system. The dynamic model of a wire-moving robot was analyzed in this paper. A sliding mode controller based on Hurwitz stability was designed for the robot to keep it balance on a tightrope. Then computer simulation based on MATLAB software was achieved to illustrate the validity and applicability of the controller. At last, though comparison with the controller build based on other method before, it was proved that this kind of controller had better control effect.