The growing number of space debris threatens the flight safety of manned space station and satellites. In 2011, NASA proposed to use a continuous ground-based laser station irradiation to change orbit of space debris. The photon pressure is slight but durable, and accumulates to a considerable orbit change. The orbit change will avoid the possible collision. This method does not bring the space debris back into the atmosphere, but to prevent the collision. For the method, the spacecraft is no need to maneuver for space debris collision avoidance, and the mild photon pressure just need median power laser generator. Thus, the method gets widespread concern in recent years. However, the head-on irradiation control law in this method is simple, but not the best. It is necessary to design the optimal control law. Through the establishment of two point boundary value model to maximize the distance between two objects for control objectives, and then solving the optimality conditions, the optimal control problem is solved. The simulation shows that the modified control law has been optimized to improve the performance of collision avoidance, and the degree is very significant, concerning to the initial orbit of the space debris to avoid collision, it is usually 25% to 125%.