In this study, a tethered satellite system is considered as a distributed parameter system. By using the extended Hamilton's principle, the tethered satellite system can be described by three partial differential equations and six ordinary differential equations. To suppress the vibrations of the tether and satellites, six boundary control laws are employed based on Lyapunov's direct method. The closed-loop system is also proved to be uniformly bounded. Simulations are conducted to illustrate the performance of the closed-loop tethered satellite system.