We present a simple and accurate method of determininy the optical constant and physical thickness of the photoactive layer in a polymer solar cell. The applicabilities of the physics models including Forouhi-Bloomer and Lorentz-Oscillator models in transmission curve fitting are compared. This method is used to calculate the optical constants and film thicknesses of poly(3-hexylthiophene) (P3HT) /6,6-phenyl C61-butyric acid methyl ester (PCBM) and poly2- methoxy-5-5(2'-ethyl-hexyloxy)-1,4-phenylenevinylene(MEH-PPV)/PCBM bulk heterojunction, The calculated transmission curves fit to the experimental ones well. The results accord with those reported in the literature and from the step profiler, and their error is less than 4%. The optical constant and the physical thickness of polymer solar cell after the optimization process including thermal annealing and adding high-boiling-point additive are studied, and the results are consistent with the voltage-current characteristics of the cell. This method is suited for bulk heterojunction films and can be used in polymer solar cell optimization and detection system.