On the base of perfect conductivity integral theory of diffraction gratings，after transforming the grating equation，the image equation of diffraction wave vector in Littrow mounting is given. Using the equation and in view of the power function property，the infinite series，which represents the kernel function，is transformed into the sum of a symmetrical series and a geometrical series. Compared with the primary integral method，from the view of numerical calculation，the new kernel function can reduce the computing time and improve the convergence. The extension results show that the power function character can improve convergence in the deviated Littrow mounting.