Abstract: In order to evaluate the reliability and applicability of the Empirical Orthogonal Functions (EOFs) applied in the acoustic inversion of sound speed profile (SSP) and reduce the dependence of SSP's basis on sample data, a theoretical methodology was proposed for the achievement of the basis in the expansion of SSP. By analyzing the oceanographic dynamics which was the cause of the SSP's change, a theoretical basis named the Hydrodynamic Normal Modes (HNMs) was derived from these dynamics equations. The orthogonal basis for SSP derived from HNMs was as good as that from EOFs, while it had less dependence on sample data. HNMs method had a clear physical meaning, and could tell the physical parameter determined the basis for SSP, which made it possible to analyze and evaluate the veracity and applicability of EOFs.