The method to stabilize the second harmonic generation (SHG) using type Ⅱ phase matching is put forward. The theoretical and numerical analysis has been done. Numerical simulation on type Ⅱ phase matching SHG course in KTP shows that when the fundamental frequency (1ω) light is incident in the SHG crystal deviating the optimal polarizing angle, the second harmonic output (2ω) intensity changes periodically with the length of SHG crystal length increasing. With this kind of periodically changing, when input 1ω intensity fluctuates, the output 2ω intensity curves converge at a certain value of the SHG crystal length. At this point the fluctuation of output 2ω intensity becomes much smaller than that of the input 1ω intensity. With the deviating angle being confined to a certain value, when the input 1ω intensity increases, the required nonlinear crystal length to stabilize 2ω intensity decreases and the energy conversion efficiency does not change. With the input 1ω intensity being confined to a certain value, when the deviating angle increases, the required nonlinear crystal length to stabilize 2ω intensity also decreases and the energy conversion efficiency remains high.