Cadmium sulfide (CdS) exhibits remarkable light absorption capabilities and is widely employed in photocatalytic reduction of CO2. Nevertheless, the role of crystal facet effects on the micro-scale mechanisms governing CO2 conversion on CdS remains elusive. This study theoretically investigates the electronic properties of hexagonal-phase (101), (001), and cubic-phase (111) CdS surfaces modified with Diethylenetriamine (DETA). From a microscopic viewpoint, it elucidates the unique bonding characteristics of CO2 on different exposed facets of DETA/CdS and the formation mechanisms leading to products such as CO, HCOOH, CH2O, CH3OH, and CH4. Our findings reveal that the DETA/CdS (101) surface is the most stable, effectively adsorbing hydrogen and CO2 at varied Cd sites with a high selectivity towards CO production, thereby showing promise for syngas generation, albeit with potential yields of formic acid and methane. Conversely, DETA/CdS (001) and (111) primarily facilitate the reduction of CO2 to CH4. These discoveries offer theoretical insights into photochemical experiments involving CO2 reduction on CdS, shedding light on the influence of crystal facets on reaction pathways.