Abstract: The sound speed gradient usually varies with seasonal change in the water layer near sea surface. The acoustic propagation losses in the near surface layer are estimated for the different sound velocity profiles in typical mid-depth sea areas of the South China Sea in spring, summer, autumn and winter respectively. The characteristics of acoustic propagation losses for different types of sound speed profiles in the spatial and frequency domains are analyzed when the horizontal distance between the source and receiver is within 12km. The results show that, due to the negative gradient phenomenon which results in acoustic shadow zone, the acoustic propagation losses in the spring and the summer reach 80dB at the horizontal distance about 1km. While in the autumn and the winter, it is typical that the surface channel phenomenon exists, so the acoustic propagation loss is less than the spherical expansion loss at the frequency above the cutoff frequency when the source is located in the mixed layer near the sea surface. However, the surface channel phenomenon is not obvious when the source depth is greater than that of the mixed layer.