The present invention relates to a method for selecting sound source frequency, used in the cased-hole dipole shear wave acoustic log. Said invention is characterized by that said method includes the following steps: firstly, utilizing acoustic moveout curve to give out compressional wave velocity of said well: then utilizing velocity field equation under the column coordinate and boundarg connection condition under the condition of cased-hole and adopting three-dimensional velocity-stress finite difference method to numerically simulate the dipole acoustic all-wave wave field of oil-water well, then utilizing the quick and slow flexural waves obtained by means of numerical simulation under the condition of that the circumference of cased-hole has abonormal stress to make dispersion analysis; then utilizing the freguency of cross point of quick and slow flexural waves to define main frequency of sound source.