A novel wavelength division demultiplexer (WDDM) based on the multi-mode interference effect (MMI) in 2D photonic crystal is presented, and it can separate three wavelengths effectively. This device takes 1310 nm/1500 nm/1550 nm for example here, and it can be extended to other wavelengths by changing the structure parameters. In addition, it can be also used to realize the separation for 1310 nm/1550 nm alone. The structure of WDDM composes of three MMI regions, which can make two wavelengths get through and one wavelength reflect back. The transmissions are above 91%. At the same time, by virtue of the plane wave expansion (PWE) and finite-difference time-domain method (FDTD), we simulate the performance.