On the basis of the spectral information, a theoretical model is developed to treat the short pulse laser generated from an active continuous wave (CW) injected ring cavity with Yb3+:YAG amplifier. Starting from rate equation, the equation describing the short pulse laser is obtained. As a computable model, it takes into account the pump absorption saturation and the laser reabsorption. By this model, the short pulse laser properties under different cases are analyzed. The results show that in order to achieve the good pulse laser, the seeding frequency and intensity as well as the amplifier length should be optimized.