In this paper a more simple and flexible analytical method for indexing X-ray powder patterns of monoclinic substances by using computer is proposed. The lattice parameter b can be obtained from the repeat number m of δ value of every pair of diffraction lines in δ-m chart and their linear relationship. By the reducing procedure the reciprocal lattice is projected on the a*-c*plane and the angle of each pair of reciprocal vectors can be calculated from the Qnkl data of each triplet of reciprocal points. According to the repeat number M in β-M chart the monoclinic angle β can be obtained and then the lattice parameters a and c can be calculated. Finally the indexing of the entire diffraction patterns is made by iterative correction method for indexing powder patterns. The computer program of the analytical method is given. The indexing of two monoclinic substances nickel acetate hydrate (C4H6NiO)4·4H2O) and potassium tungstate (K2WO4) has been made by using this method, the result proves that the method is fairly applicable.