The photopromoted methoxycarbonylation of olefin with methyl formate catalyzed by Co(OAc)2 at ambient conditions has been carried out. The results indicated that the reaction activity increased with the increasing temperature. Methyl formate decomposed into CO and CH3OH firstly under irradiation, and then the methoxycarbonylation of olefin proceeded under catalysis of Co(OAc)2.The mechanism of methyl formate participating in the methoxycarbonylation is verified by the IR analysis and the labeling experiments of CD3OD and CH318OH.