Abstract: The problem using arrival time difference between direct wave and sea surface reflection wave to invert the sound speed profile (SSP) in deep-ocean is studied based on ray theory. The gravest empirical mode (GEM) method is employed to organize the historical hydrographic data for SSPs inversion. Experimental data of the winter survey in the South China Sea in 2016 are analyzed. It is shown that the inverted SSP is fitted well with the measured SSP, and the maximum value of the absolute error is 2.15 m·s^-1 in the shallow water layer (0 ~ 710 m).