The trace amount of CO in H₂-rich gas poisons Pt electrode when it is adopted as feedstock for proton-exchange-membrane fuel cells. Preferential oxidation of CO (PROX) is a promising approach to selectively oxidize the trace amount of CO while keeping H₂ unoxidized. Catalyst plays important roles in PROX. To date, enormous catalysts have been developed for PROX. Summarizing the catalysts developed for PROX and unveiling the reaction mechanism could definitely advance this research field. Herein, in this review, according to the nature of the active sites on the catalysts, we classify the catalysts into group VIII metal-based catalyst, group IB metal-based catalysts, group VIII-group IB bimetallic catalysts, transitional metal oxide catalysts as well as others, describe the progress of the catalysts in PROX in the latest five years, and extract the underlying reaction mechanism, with the aim to provide guidance for the rational design of efficient catalysts in the future.