To obtain ultra small and ultra dense wavelength division demultiplexer, an intelligent inverse design method for multi-channel on-chip micronano wavelength division demultiplexer is proposed. The sequential quadratic program（SQP） algorithm and finite element method（FEM） are introduced, supplemented by interpolation, shaping, and binarization techniques, to achieve reverse design from functional requirements to specific structures. Through this method, multiple types of wavelength division demultiplexer s have been successfully designed and experimentally validated. The experimental results show that the designed wavelength division demultiplexer performs well in terms of size, transmission efficiency, and extinction ratio.