The invention relates to a method for coating films with large area offline. The method is characterized by including that a, precursor solution is gasified in a gasification device; b, the precursor gas obtained in the step a is carried by carrier gas to pass a pipeline to be delivered to a coating-film reactor; and c, the precursor gas sequentially passes a group of small gas cavities, being communicated with one another, of a gas feeding cavity to be mixed, and the evenly-mixed precursor gas is used for coating films on on-line glass in a coating-film chamber after passing a coating-film discharging port. By the aid of the proper precursor gas, one or two functional films with uniform thickness and no obvious optical interference fringes are evenly coated on the surface of moving large-size hot glass.