The concept of separated flow and numerical calculation of two-dimensional unsteady heat transfer were used to develop a model of the temperature change of flue gas flowing out of finned tube bundles. A method for comparison was designed to improve the separated flow model and the transient experiment method. The heat transfer coefficient of air flowing transversly through crossed finned tube bundles was established in this way and experimental study on the coefficient was made with the constant wall temperature method. The result was satisfactory.