Three kinds of simulating models for optical pulse chirped amplifier (OPCPA) are introduced at first. Then the three-step way to determine the length of the nonlinear crystal for most stable output is put forward. In this method, all the there models are used. As the more complex models are used, the ranging length for simulation becomes shorter. So it is efficient and accurate to useing this method to determine the optimized nonlinear crystal length. Then the way is also studied to introduce some phase mismatching in OPCPA to detune the nonlinear crystal length for most stable output. Simulating results show that as the mismatching angles increases, the nonlinear crystal length for most stable output becomes longer. So it is suggested that the nonlinear crystal be manufactured longer than designed. Most stable output can be obtained by introducing some phase mismatching in the experiments.