The conceptions in microwave engineering are used to modeling the light transmission process in fiber Bragg gratings,yielding the corresponding transfer matrix in the microwave network grating model. The relationship of this novel transfer matrix and the traditional transfer matrix in piecewise uniform grating model is clarified. Combining the modified effective index method,a novel method for calculating the complex reflection spectrum is proposed. The MT matrix in this method is concise and a good exactitude of this method is confirmed by numerical simulation.