Wavelength beam splitter is one of core components in the field of optical communication. In order to meet the design requirements of increasingly complex micro-nano devices, a new intelligent inverse design method which combining the Finite Element Method (FEM) with the Method of Moving Asymptotes (MMA) and the Material Interpolation Method (MIM) has been presented, which has a faster calculating speed and a lower arithmetic complexity, and adapt to the micro-nano photon devices design. This algorithm can complete the optimal design according to the objective function given by the designer, with low professional requirements for designers and wide range of adaptations. Based on this algorithm, a three-channel chip-integrated wavelength beam splitter with footprint of 2μm×2μm is successfully designed. Its transmission rates of the three channels are 47.5%, 53.4% and 52.5%, respectively, and the extinction ratios are 36, 9.6 and 20. Which provides a new idea and reference for design of micro-nano devices.