The paper intends to go theoretically into the mechanism of physical chemistry of the preparation of Mo 2C film by using MOCVD.It has proved,by the result of experiment,in the process of the preparation of Mo 2C,that the two-step mechanism which Mo(CO) 6 should follow:firstly,Mo(CO) 6 is absorbed by the fundamental bottom and does a phase change;and secondly,it is decomposed.the process of decomposition of Mo(CO) 6 is a sequential one,which is finally carbonized into Mo 2C film from Mo.By applying Gibbs functional smallness principle to the calculation of Gibbs function of chemistry reaction in MOCVD when it deposits Mo 2C film,the best temperature of preparing film is probed.