The effects of different photoionization parameters are systematically studied. When the nitrogen-oxygen ratio, photon absorption cross section or photoionization efficiency coefficient are reduced, the streamer branching occurs earlier in three cases after reducing the photoionization rate. These results imply that the streamer shows higher possibility of branching if the photoionization rate decreases. When the streamer propagates in a non-uniform electric field region and the photoionization rate decreases to a certain value, it is believed that the seed electron distribution is more susceptible to random fluctuations. It will lead to instability in the space charge layer of streamer, thus causing the streamer to branch. Hence it is proposed that streamer branch will be triggered more easily if the photoionization rate in the streamer head decreases, in the case without considering other seed electron sources.