The thickness of the active layer is limited by its low carrier mobility in the polymer solar cell composed of the blend bulk-heterojunction formed by conjugated polymer as donor material and fullerene as acceptor material, which can affect the light absorption in the polymer solar cell. TiO2 inserted into polymer solar cell as optical spacer can redistribute the electromagnetic field inside the device and enhance the light absorption of it. In this paper, light intensity and absorption inside the devices with different thicknesses of P3HT:PCBM layer and TiO2 layer are calculated based on transfer matrix method. Theoretical analysis shows that inserting 10 nm TiO2 into the device can increase 16.3% light absorption, simultaneously thinning the active layer by 25 nm and the thickness of active layer will not apparently reduce the dissociation rate of the excitons. With the device structure of ITO (100 nm)/PEDOT:PSS (40 nm)/P3HT:PCBM/TiO2/LiF (1 nm)/Al (120 nm), the optimal thicknesses of the active layer and TiO2 are 75 nm and 10 nm respectively, which is confirmed by the experimental results from the devices with three different structures.