With the rapid advancements in technologies like 5G, big data, and AI, Internet data traffic is rising exponentially, driving an urgent need for high-speed data interconnects in data centers. Large-scale models such as ChatGPT require robust interconnect systems to facilitate high-capacity chip communication. However, traditional electrical interconnects face limitations, including signal loss, power consumption, and reliability issues, particularly for centimeter- to meter-scale board-level distances. Optical interconnects, with their high bandwidth, fast data rates, low power needs, and immunity to electromagnetic interference, are increasingly preferred for next-generation systems.Polymer waveguides, compatible with both Printed Circuit Boards (PCBs) and optical fibers, support high-density, high-speed connections with low propagation loss. Polymer-based optical backplanes currently reach transmission rates of up to 112 Gbps per channel. To increase capacity further and achieve transfer rates of 400 Gbps, 800 Gbps, and up to 1.6 Tbps, Wavelength Division Multiplexing (WDM) is a promising technology, especially in the O-band (1 260~1 360 nm). Among WDM options, Coarse Wavelength Division Multiplexing (CWDM) offers cost benefits in equipment and maintenance. CWDM systems rely on (de)multiplexers(MUX) for separating and combining different wavelengths. Common (De)MUX designs include arrayed waveguide gratings, planar concave gratings, and microring resonators. The cascaded Mach-Zehnder Interferometer (MZI), due to its low insertion loss, wide bandwidth, and low crosstalk, is widely used for CWDM devices. While CWDM research is extensive, studies on polymer-based devices remain limited. Polymer waveguides exhibit lower refractive index contrast and larger feature sizes, providing reduced sensitivity to geometric variations and allowing larger fabrication tolerances. Polymer waveguides are also straightforward to manufacture through spin-coating and UV lithography at room temperature, which helps minimize costs. With polymer waveguides validated for high-speed interconnect backplanes, CWDM integration can enable cost-effective all-polymer optical circuit boards with enhanced communication capacity and data rates. This work presents a four-channel CWDM device based on a two-stage cascaded MZI structure in polymer waveguides. The MZI design uses Multimode Interference (MMI) couplers as power splitters, which are insensitive to wavelength and polarization, contributing to stable performance. By optimizing the MMI′s parameters, we achieved consistent performance, with insertion loss and imbalance variations kept within 0.1 dB and 0.4 dB, respectively, even with a 10% variation in refractive index contrast. This improves the device′s resilience to refractive index changes from temperature fluctuations. To further reduce insertion loss in phase arms, we introduced an offset at the junction between curved and straight waveguides, achieving a device loss reduction of approximately 1.5 dB. Simulations and experimental results demonstrate that the device achieves an insertion loss below 3 dB in the O-band, with crosstalk below -28 dB across a 3-dB bandwidth. Experimental verification, achieved through UV lithography, confirmed an excess insertion loss of 2.7 dB in the O-band, a 3-dB bandwidth of 20 nm, and channel crosstalk consistently under -20 dB. The device′s polarization-dependent loss remains below 1 dB. Thermal stability tests indicate a spectral shift of less than 1.8 nm at 150 ℃, suggesting suitability for high-speed board-level optical interconnects where operating temperatures can vary. In conclusion, this study presents the design and fabrication of a polymer-based four-channel CWDM device with a two-stage cascaded MZI structure, optimized for low insertion loss, wide bandwidth, and low crosstalk. This CWDM solution holds potential for scalable optical interconnects in data centers, offering a practical pathway toward low-cost, high-speed all-polymer optical circuit boards.