In the design process of a novel photonic crystal 1×2 power splitter, the coupling of three parallel photonic crystal waveguides can be considered as a multimode interference system in the two-dimensional square photonic crystal. The positions of output waveguides are decided by the positions of periodic twofold image which are formed by the self-imaging effect of multimode interference.The transmission characteristics of the splitter are investigated by using the finite-difference time-domain method. The reflective losses obviously decrease by shifting a dielectric rod in the junction between the multimode waveguide and single-mode output waveguide, and microcavity is constitued, which varies and matches the mode field of the coupling zone. The results show that the transmittance of this splitter can be as high as 99.04% for λ=1.55 μm when the shifted dielectric rod is in the position of 1.85a from the junction between the multimode waveguide and single-mode output waveguide.