Abstract: A new sliding mode control(SMC)method based on stable analysis for a class of underactuated systems is presented.In this method,an interim variable is defined by combining one variable of each subsystem.The interim variable is used to construct the sliding mode function and the total control law can be derived to guarantee the equilibrium point is reachable in finite time.Using LaSalle invariance theorem,it can be proved further that the equilibrium field only includes one equilibrium point and the equilibrium point is asymptotically stable.The simulation results validate the conclusion.