Copper-based catalysts have been recognized as promising candidates for electrochemical conversion of CO2 to value-added chemicals and synthetic fuels. Yet, the challenges of high overpotential and low product selectivity have motivated the rational electrode engineering. In the present work, we prepared CuS catalysts using different sulfur precursors, and we aimed to elucidate the precursor-dependent effect on their structure–property–activity relationships for electrochemical CO2 reduction. The different sulfur precursors exhibited varied S release rates in hydrothermal synthesis, which had induced distinct surface morphological features and diverse sulfur vacancy concentrations, and the intrinsic catalytic activity and product selectivity would be affected. The desired CuS-TU catalyst synthesized using thiourea as the sulfur precursor featured a flower-like morphology and had the highest sulfur vacancy concentration. The nanoflower morphology offered expanded space and considerable undercoordinated sites for facilitated interfacial mass transfer in electrochemical CO2 reduction. Density functional theory calculations confirmed that the abundant sulfur vacancy played an important role in strengthening the adsorption of the *COOH intermediates on the surface, which promoted CO production via the *COOH pathway. The CuS-TU catalyst therefore exhibited a relatively higher CO selectivity of 72.67% at −0.51 V vs RHE. These findings will provide more insights into improving the electrochemical CO2 reduction performance of copper-based catalysts by structure engineering.