By using the irreducible tensor method of representing SHG coefficients, we have proposed in this paper a general transformation formula for SHG coefficients of the macroscopic crystal and those of its microscopic anionic groupings. It thus leads to a series of calculation formulas suitable generally for calculating the contributions of the anionic groupings of different orientations in the crystal to the macroscopic SHG coefficients. It has been shown that this method can be used conveniently to calculate the SHG coefficients of various iodates. The agreements of our calculation values with the experimental ones are quite satisfactory. Our work has provided a dear stero-structural criterion for searching of new types of SHG materials.