A new compact microfluidic chip based on multimode interference (MMI) waveguide is designed and simulated, which can be used to measure the refractive index of liquid in micro-channel through the interference of multiple optical guide-mode waves. The change of concentrations or composition in liquid leads to different refractive index, the change of refractive index can be measured by the output power of MMI waveguide, and then the information of liquid can be determined in details, such as the composition and concentrations. The sensitivity of refractive index can reach 0.0001 according to simulation. Two pieces of chips are proposed for different flux fluids, which can be applied to the microfluidic chemical analysis chip.