For the problem of inhomogeneity of the output optical power of multiple output ports, the paper designs a 1×4 Multimode Interference (MMI) coupler based on Silicon-On-Insulator (SOI) and proposes a new method to optimize its homogeneity. A tapered waveguide is used at the input/output of the coupler. To improve the uniformity of the MMI coupler, an unequal-width design is used for the tapered waveguide at the output. After optimization, the uniformity of the four output ports is as high as 0.007 4 dB, while the total insertion loss is only 0.058 dB. According to this method, the transmission constant of the output, avoid crosstalk between waveguides, and achieve an improvement in the uniformity of the output port of the MMI coupler.