In this paper, a two-stage control law is proposed for posture stabilization problem of robot fish. The two stages includes: fast position approaching and accurate posture adjusting. In the first stage, a modified proportional navigation guidance law is used such that the robot fish swims fast toward the target point. In the second stage, a time-varying feedback control law is used to adjust its position and direction accurately. Finally, the control law is applied in a robot fish pushing ball experiment on the global vision experimental platform. The results show that the control law can be used conveniently and effectively.