Based on the structures of GaAs/InAs-GaAs/ZnSe P-i-N quantum dot solar cells, according to the optical principle and diffusion theory, mathematic model describing the relationship between photogenerated electron current density and thickness of layer is proposed, and the effect of the quantum dot layer on the characteristics of solar cell is analyzed quantitatively for improving the power conversion efficiency of quantum dot solar cells. Simulations show that the optimal thicknesses of P(GaAs) and N(ZnSe) are 1541 nm and 78 nm respectively when the i layer thickness is 3000 nm, and the power conversion efficiency of solar cell is 20.1% at a single wavelength; At the same time, the volume of quantum dot and the temperature affect* I-V* property of quantum dot solar cell, and the value of open voltage reduces with the increase of the volume of quantum dot and temperature, so that the power conversion efficiency will be reduced.