The theoretical model of the heat transfer in the photovoltaic thermal (PV/T) collector with different parallel cooling channels is established to evaluate the effect of parallel cooling channels on the thermal performance of the PV/T collector. To design the structure of the parallel cooling channels, the influences of structural paremeters are discussed. The numerical results indicate that with the number of the subchannels of 10, the pressure drop of the cooling channels is the lowest. Meanwhile, the temperatures of the photovoltaic modules are low and evenly distributed. Moreover, the I-type configuration of the inlet and outlet is the best choice, since the pressure drop of the cooling channels is low and the thermal performance of the PV/T collector is excellent. With the increasing diameter ratio between the main channel and the subchannels, the temperature of the photovoltaic modules is decreasing and the more evenly distributed. Moreover, the figure of merit of the cooling channels with D/d = 4 could be 50% larger than that with D/d = 2.