According to the latest experiments about the temperature effect of organic magnetoresistance，we employed a drift-diffusion equation and take into account the temperature influence on mobility and spin relaxation to investigate spin polarization and magnetoresistance of the organic semiconductor device. We found that the magnetoresistance in low temperature region decreases faster than that in high temperature region. The change of magnetoresistance with temperature is mainly dependent on spin relaxation time of organic layer. Finally，the theoretical calculation was compared with experimental data and consistency between them was obtained.