A three-mode division multiplexer and demultiplexer based on two-dimensional square-lattlie photonic crystal with wavelength of 1550 nm was designed. Two asymmetrical directional couplers are included, which can achieve modes conversion function of TE0, TE1 and TE2 modes. In order to avoid phase mismatching in bus waveguide, taper structure is applied at waveguide junction which can reduce the insertion loss effectively. Plane waves method and finite difference time domain methods are used to simulate the performance. Numerical simulations show that the designed device has the potential for high-capacity mode division multiplexing optical communication systems with insertion loss less than 0.14 dB and mode crosstalk less than -20 dB.