Efforts are made to obtain a novel multimode optical power splitter based on asymmetric Y splitter by means of beam propagation method (BPM). As lack of uniformity for multimode light power in waveguides, a special asymmetric Y branch on curves is brought into the structure to reach an evenly divided power output. Relation between splitter parameters and the output power has been analyzed. A series of equations for the splitter structure design are obtained. Both 1×4 and 1×8 power splitter designs with a 62.5 μm and 120 μm waveguide width respectively are provided with an overall length of 7.6 mm and 21.7 mm, respectively. The calculation results show that these splitters exhibit an excellent splitting uniformity in 1.31 μm window. The calculated insertion loss for the two designs are 6.05 dB and 9.06 dB with the polarization-dependent loss to be 0.2 dB.