A novel six-mode multiplexer with large effective area and graded-in dex distribution is proposed.Moreover,the performance of the mode division multiplexer is inves tigated using the beam propagation method and full-vector finite element method.The results show that the mode division multiplexer with graded-index distribution,large effective area,low nonlinear coefficient,high coupling efficiency and broadband is achieved.The mode divisi on multiplexer can perform multiplexing and demultiplexing operation of six modes.From 1450nm to 1650nm,the coupling efficiency of the mode division multiplexer is better than －0.449dB,t he coupling efficiency of LP₀₁,LP11a,LP_11b,LP21a and LP_21b are respectively better than 0dB,－ 0.018dB,－0.018dB,－0.449dB and －0.036dB,it presents a flattened profile.In the range of C band,the coupling efficiency of mode division multiplexer shows flattened characteristics better, and the flatness of all modes is better than 0.066dB.The effective area of all modes is 14.9times larger than that of the standard single-mode fiber at 1550nm,and t he nonlinear coefficient is much smaller than that of the standard single-mode fiber.The design of the multiplex er is an effective method to achieve low nonlinear coefficient,high coupling effic iency and broadband for mode division multiplexing.