After analyzing the type I phased matched coupling equations for second harmonic generation (SHG) course, it is found that when the input 2ω signal intensity and the preliminary phase difference θ(0)=2(0)-21(0) change, the output 2ω intensity varies along with the length of SHG in different way. When the input 2ω signal intensity is not zero and θ(0)=2(0)-21(0)≠±π/2, part of the energy oscillates between 1ω and 2ω. It is proposed that this kind of phenomenon can be used to improve the stability of the output 2ω intensity. Simulation proves that the method of injecting 2ω signal can greatly improve the stability of SHG course. Further simulation shows that length of SHG crystal changes as the input 2ω intensity and phase difference between 1ω and 2ω changing.