Abstract: This paper addresses time-invariant gait planning and finite-time nonlinear control strategy for a five-link,four-actuator planar biped robot to realize dynamic walking.Nonsmooth character and hybrid models,composed of under-actuated phase and fully-actuated phase,are uncovered,and the collision model is derived.Based on the concept of virtual constraint,we present a method to design output functions analytically for time-invariant gait,and design a feedback linearization controller which transforms the system into a loop of double integrator.Then,the outputs can be stabilized to zero within a step by a finite-time controller.Simulation results indicate that a stable limit cycle of dynamic walking is achieved,the planned walking pattern is realized,and the effectiveness of the proposed method is verified.