Polymer solar cell with excellent performance is currently composed of the blend bulk-heterojunction formed by conjugated polymer as donor material and fullerene as acceptor material, among which P3HT and PCBM are most widely used. For the photoelectric thin film device, the optical and the electric characteristics of the active layer can affect the device performance directly. The optical constants of the active layer are obtained primarily based on the Forouhi-Bloomer model. According to the Fresnel coefficient matrix, the numbers of photons absorbed by different active layer thick devices are investigated by calculating electromagnetic field distributions inside this structure. To make a specific analysis of the thickness effect on dissociation probability of bound pairs without external excitation, the model of Onsager-Braun is adopted. Theoretical analysis shows that the active layer with a thickness of 100 nm can maximize photon absorption by the device without reducing the dissociation probability of excitons obviously. As a result, an optimal thickness of the active layer is about 100 nm, which is confirmed by the theoretical and the experimental results from the device with the structure of ITO/PEDOT/ P3HT:PC60BM /LiF/Al.