The invention relates to a method for trapping and purifying CO2. A coal-to-methanol synthesis gas purifier is utilized; a low-temperature methanol washing process is adopted; sulfur-free medium-pressure and low-temperature methanol rich liquid is taken as a raw material; an industrial liquid CO2 product of meeting the standard requirements is obtained by separation, purification and cooling; the method is applied to an oil exploitation gas drive technology to improve the oil recovery rate; meanwhile, CO2 is sealed and stored; and emission reduction of CO2 is achieved when certain economic benefits are obtained. The method has the advantages that the trapped and purified liquid CO2 product is applied to the oil exploitation gas drive technology to improve the oil recovery rate; certain economic benefits can be obtained; meanwhile, sealing and storing of CO2 are achieved; emission reduction of CO2 is achieved; and the method has great social benefits.