Abstract: The surface Helmholtz integral equation may effectively works out sound radiation and scattering field of an object. However it can not get satisfactory results at all frequencies. When the wave number k is close the eigenvalue (resonance frequency) of the interior problem of the target, it is shown that there is no unique solution and it affects seriously the accuracy of solutions. In this paper, according to Combined Surface Helmholtz Integral Formulation (CHIEF), the least-squares orthonomalizing procedure is used effectively to get the exact solution of the sound scattering by a target at resonance frequencies. The sound scattering by circular and elliptical cylinders were calculated as an example and how to make the interior points for additional calculating were presented.