It studies the optimization of charged particle optical system and proposes to separate the optimization of aberration from the optimzation of field in lens. In the optimization of aberration, the implication constraints are considered. In this optimization, it needs to do some simple integral calculations so that the calculation of field which will takes a lot of time is avoided. In the optimization of filed in lens, the mathmatical model is just the same as in other electromagnetic system. Therefore, the optimzation methods of electromagnetic system can be used to optimize the field in lens. In the end, two examples for optimization are given to verify the new method.