Highly crystalline carbon nitride polymers have shown great opportunities in overall water photosplitting; however, their mission in light-driven CO conversion remains to be explored. In this work, crystalline carbon nitride (CCN) nanosheets of poly triazine imide (PTI) embedded with melon domains are fabricated by KCl/LiCl-mediated polycondensation of dicyandiamide, the surface of which is subsequently deposited with ultrafine WO nanoparticles to construct the CCN/WO heterostructure with a S-scheme interface. Systematic characterizations have been conducted to reveal the compositions and structures of the S-scheme CCN/WO hybrid, featuring strengthened optical capture, enhanced CO adsorption and activation, attractive textural properties, as well as spatial separation and directed movement of light-triggered charge carriers. Under mild conditions, the CCN/WO catalyst with optimized composition displays a high photocatalytic activity for reducing CO to CO in a rate of 23.0 µmol/hr (i.e., 2300 µmol/(hr·g)), which is about 7-fold that of pristine CCN, along with a high CO selectivity of 90.6% against H formation. Moreover, it also manifests high stability and fine reusability for the CO conversion reaction. The CO adsorption and conversion processes on the catalyst are monitored by in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), identifying the crucial intermediates of CO, COOH* and CO*, which integrated with the results of performance evaluation proposes the possible CO reduction mechanism.