A novel Si four-wavelength multiplexer (MUX) for CWDM system composed of two types of Mach-Zehnder (MZ) MUX, and a polarization splitter rotator (PSR) based on a tapered asymmetric directional coupler (ADC) and TE₁-TM₀ mode converter (MC) is proposed and experimentally demonstrated. To make the ADC fabrication-tolerant and broadband, a new design method is proposed, in which waveguide width dependence is taken into account. Although due to the waveguide width change during the fabrication process, the loss of the fabricated ADC is relatively large, a proof-of-concept four wavelength multiplexing is experimentally demonstrated.