Based on supermodes derived from coupled mode theory,we present the formula to analyze dual core fiber gratings with rectangle-shaped index change,and give its general solution. To the same core with same synchronous uniform grating model,we present a simple express of the solution for each core that can be straightly used in calculation. In this model,the effect of grating is shifting resonant wavelength and adding different additional phases for different wavelengths and it could be used for adjusting the output spectrum of the dual core fiber,and controlling on its additional phase. To the model that gratings are different in index change,we give a numerical simulating result,and discuss its impact on performance.