The study of tidal changes is of great significance in marine engineering, marine mapping as well as marine transportation. Because nearly all tide gauges are located in the coastal waters, previous studies mainly focus on tidal changes in the shallow waters. In the deep sea, due to the lack of long-term high-frequency sea level observations, tidal changes are remained unclear. Based on the tide gauge observations and satellite altimeter data, non-stationary tidal harmonic analysis toolbox S_TIDE to extract the long-term trend of amplitudes of four major constituents in the South China Sea is first used in this paper. Results show that in most areas of the South China Sea, the amplitudes of four major constituents are stable and have no significant positive or negative trends. In minor areas of the South China Sea, the amplitudes of four major constituents have significant trends. The largest positive trends can reach 2.91 mm/a and the largest negative trends can reach 3.50 mm/a. The long-term trends of tidal amplitudes in this area may be related to the change of the surface expression of internal tides. The tides observed by satellite contain not only barotropic tides but also the surface expression of internal tides. The internal tides as well as their surface expression in the South China Sea are strongest in the world. The changes of ocean stratification can influence the generation, propagation and dissipation of internal tides as well as their surface expression and eventually induce the long-term trends of tidal amplitudes in the South China Sea.