The conventional WDM are usually large in size

which has become the bottleneck of the photonic integration.This paper designs a type of three-wavelength（1 310nm

1 490nm and 1 570nm）photonic crystal WDM by utilizing the multimode interference effects of photonic crystals.The simulation results show that the insertion loss of this device for the three wavelengths is all less than 0.45dB

and the channel isolation is all larger than 15dB.In addition

its size is only 37#m $13#m.easy to be optically integrated and superb in performance.