This paper proposes and fabricates a coarse wavelength division multiplexer (CWDM) chip based on a grating-assisted backward coupler. The CWDM is composed of three gratings with different reflection wavelengths cascaded together. The introduction of the grating-assisted backward coupler structure can avoid the use of discrete circulators or the 6 dB additional loss caused by on-chip beam splitters, while effectively preventing the interference of reflected light on the signal light. By optimizing parameters such as grating width and the number of grating periods, and performing apodization design on the gratings, a three-channel CWDM is designed and fabricated on a silicon-on-insulator platform. Experimental results show that the on-chip insertion loss of the device is approximately 2.5 dB, the channel crosstalk is less than -24 dB, the spectrum presents a distinct box-shaped response, and the chip area is only 0.6 mm × 1.5 mm. This paper successfully breaks through the technical bottlenecks of large size, small bandwidth, and difficulty in obtaining box-shaped spectral response of CWDM devices based on common structures. The developed chip is expected to be widely applied in communication systems such as metropolitan area networks, fiber-to-the-home, data centers, and on-chip interconnection systems.