A piecewise control strategy is proposed for the planar four-link underactuated system with a passive second joint. Firstly, a mathematical model of the system is built, and it is degenerated by controlling the first link maintaining at the initial state. Then, the angle constraint relationships are obtained by employing the integral characteristics of the planar link system with a first passive joint. The target angles of actuated links are calculated by using particle swarm optimization algorithm based on the angle constraint relationships and target position. Finally, the controllers designed by employing Lyapunov functions achieve the control objective from an initial position to a target position. Simulation results show the effectiveness of the proposed control method.