Existing computed results for formic acid (HCOOH) dehydration on Pt catalysts cannot explain the facile CO poisoning effect. By DFT calculations, this work studied HCOOH dehydration on a Pt(111) surface assisted by co-adsorbed HCOOH or H2O molecules. The calculated results show that CO is formed via intermolecular dehydration pathways. The newly proposed mechanisms provide a valuable input to understand the degradation problem of Pt catalyst in HCOOH decomposition.