Designing stable and efficient control methods for walking and running of biped robots has been a challenge over the past years. The goal of this paper is to design a finite-time controller to stabilize the running gait of a compliant leg biped robot. The biped model consists of two springy telescopic legs with flat feet. The robot model has five actuators including a torque actuator at the hip joint, two torque actuators at the ankle joints and two force actuators at the legs prismatic joints parallel to the springs. Dynamic equations of stance phase, flight phase and touchdown event are derived to build the dynamic model of a complete step of running. This robot model has passive periodic running gaits which are unstable. The controller aims to stabilize the gaits around the desired limit cycles and reject disturbances. Finite-time sliding mode control is used to design controller for the stance phase; the flight phase remains passive. Starting from various deviated initial conditions, the state space trajectory of the system with the designed control law is derived and its convergence to the desired trajectory is shown. The result of the controller is also compared with an event-based linear controller from previous references.