A completely decoupled method for subsystem controller's design was given based on the comparison equations' stability criterion of linear interconnected large scale system. This method constructs a vector Lyapunov function to ensure the stability of the comparison equations. And the controller's design is converted to solve a set of linear matrix inequalities. Then these controllers are used to stabilize the large scale system. Compared with those methods based on scalar Lyapunov function, this method reduces the control algorithm's computing and difficulties, due to independent design for each subsystem controller. The simulation results show the feasibility and effectiveness of this method.