Polymer photovoltaic cells were fabricated with the basic device structure of ITO/PEDOT: PSS/P3HT: PCBM/Ca/Al. The magnetic-field-dependent current under illumination and in darkness in these devices were measured respectively under different bias. It was found that the light current was enhanced by the magnetic field whereas the dark current was suppressed. Based on the formation mechanism of the light current and the dark current in polymer solar cell, the observed magnetic field effects were explained. The results show that the magnetic field can effectively change the relative ratio between the singlet and the triplet polaron pairs, resulting in an increase in the density of the free carriers. The concentration augmentation of photo-generated free carriers is responsible for the increase of the photocurrent. On the other hand, the interaction between the free carriers and triplet excitons leads to the decrease of the dark current. Around the open circuit voltage, the magnetic field dependent photocurrent and dark current acting together causes the magnetic field effect of light current.