In this paper, an ultra-small silicon optical multi-wavelength router is designed, which can realize the multi-wavelength routing function in optical communication window. The guided wave characteristics and the energy propagation characteristics of the light field are numerically simulated by using the Plane Wave Expansion (PWE) method and the Finite Difference Time Domain (FDTD) method, respectively. The designed wavelength router has the characteristics of high transmission efficiency, ultra-small overall size (total length about 20 μm), which can realize the routing function of 1.31, 1.55 and 1.65 μm optical wavelength at the same time.