The design of an optical add-drop multiplexer (OADM), using an asymmetric coupler and a slanted Bragg grating, is presented. By employing 3-D normal modes, theoretical analysis is made over three possible structures to choose the grating-on-wide-waveguide one. Through coupled-mode theory, the simulation of the coupling properties of the slanted grating is given to optimize its tilt angle. Moreover, by utilizing the back-reflection-peak-wavelength method, the wavelength to be added and dropped is located within the wavelength division multiplexing (WDM) signal channels while the peak wavelengths of the reflective spectrum are located outside of the WDM signal channels, improving the performance of the proposed device greatly. Simulations show that the device has a crosstalk about -30 dB and a return loss about -25 dB. Meanwhile, the increase in key process tolerance enables mass production of the device easier. The device has a crosstalk below -28 dB and a return loss below -22 dB with the tilt angle of the slanted grating fluctuating from 2.5° to 4.5°.