Based on the Fourier factorization process in Lalanne′s empirical method and Gtz normal vector method,a fast convergence solution was proposed for Fourier modal slice absorption method.The gradient vectors of analytic boundary function were used to simplify the Fourier factorization process in Gtz normal vector method,which made the caclation time for a single wavelength point reduced by 2 orders of magnitude.4 typical types of normal vector fields were built by inverse distance weighting algorithm,and the degrees of freedom in normal vector method was reduced.According to these 4 normal vector fields and the Lalanne′s method,5 types of convergence improved matrice were finally presented.By pre-selection among the 5 matrice,an optimal matix could be implemented in the caculation of Fourier modal slice absorption method to guarantee fast convergence.Calculation results demonstate that for a simple silicone structure the optimal matrix has 2 diffraction oders improved compared with the un-preferred slected matrix,and effectively improves the simulation rate of diffraction.The proposed fast convergence solution is expected to improve the efficiency for grating design and analysis/testing of integrated circuit.