From three respects such as mathematical principle,geometric stratification and numerical calculation,we have studied the convergence of rigorous coupled-wave approach (RCWA) in solving the grating diffraction problems.The approaches which can be applied to improve the convergence of RCWA are proposed.It is found that,for different cases of grating,the major factors affecting the convergence of RCWA are different.Therefore,the improving methods should be practically chosen for concrete case of grating.