In the Fraunhofer diffraction approximation, a general model for calculating the transmission grating diffraction efficiency is deduced from wave optics under the condition of far field. Diffraction efficiency of transmission grating with any cross section can be obtained from such a general model. Rectangular, trapezoidal, 5-side quasi-trapezoidal and 7-side quasi-trapezoidal cross section calculation method are established with this model. And two examples are given to demonstrate the correctness of this general model.