An O-band 4-channel wavelength division (de)multiplexer with low insertion loss and flat-top transmission spectra is presented based on the novel scandium (Sc)-doped aluminum nitride (Al1-xScxN) photonics platform, and an optimization method is proposed. The designed multiplexer was based on cascaded Mach-Zehnder interferometer (MZI) filters, using directional couplers with curved waveguides to reduce wavelength dependence. Particle swarm algorithm (PSO) was adopted to increase the efficiency of device optimization according to the characteristics of coarse wavelength division multiplexing (CWDM) applications. The transmission quality of four channels was improved by adjusting the design parameters of the device structure. For Sc doping concentrations of 0%, 9% and 23%, the designed multiplexer has 1-dB bandwidth as wide as about 15.6nm and negligible insertion loss of less than 0.1dB, showing “box-like” transmission response. The crosstalks of four channels are less than -30.6 dB.