Based on the finite difference time domain method, factors and physical mechanism of different kinds of common metal nanoparticles are studied that influence the absorption efficiency of organic solar cells. With the analysis of field distribution of localized surface plasmon resonance stimulated by metal nanoparticles, effects of metal nanoparticles in different functional layers on the absorption efficiency of organic solar cell are compared. According to Mie theory and electrical resonance effect, the relationship between the structural parameters of metal nanoparticles and the influences they have on the position and intensity of localized surface plasmon resonance is found, and an optimized design of organic solar cells is proposed. The result shows that when small and highly symmetrical metal nanoparticles are closely packed into the active layer of a solar cell, more than three times light absorption enhancement can be achieved.