With the optical interference and exciton diffusion principle, the mechanism and process in a typical organic donor-acceptor solar cell are analyzed and simulated in virtue of a model established by MATLAB software. The effect of the organic film's thickness on the characteristics of organic solar cell is analyzed quantitatively, photo-absorptivity and exciton diffusivity are optimized by confining the thickness of organic active layers. The reliability of optimized model is validated by experiment.