Based on coupled mode theory and the structure of curved waveguide coupler (CWC), the performance of planar CWC was analyzed.The result shows that the two variables,bending radius and minimum space,increase the flexibility of waveguide device design.By the modulation of effective coupling length,CWC can accommodate wider multiplexing bandwidth than parallel straight waveguide coupler (SWC) in wavelength division multiplex (WDM).The effect of bending radius and minimum space on the multiplexing bandwidth of curved coupler was studied numerically,which provided a certain theoretical basis for design and fabrication of devices.