Based on the structure of an imported impingement air blast freezer, the design parameters of the evaporator with aluminum tube and fin were predefined. The refrigerant was R717, and refrigeration capacity was 70kW. Heat transfer coefficient of air and refrigerant were calculated separately, regardless to the influence of dirt in the tube, the method for calculation heat transfer coefficient of this kind of evaporator was gained. By way of the experimental investigation on this type of evaporator, the calculation method was indicated to be reasonable.