The thickness of the oceanic crust is affected by many factors. Most of the predecessors have paid attention to the deep factors of the lithosphere such as mantle temperature and mantle source composition, and rarely to the influence of hydrothermal circulation in the shallow lithosphere on the thickness of the oceanic crust. The relationship between hydrothermal circulation and oceanic crust accretion under different backgrounds of expansion period (mid- ocean ridge, disassembly faults) and different expansion rates was studied by finite element- based numerical simulations. The results show that before the oceanic crust accretion reaches stability, the hydrothermal circulation leads to a phased thinning of the theoretical oceanic crust thickness. The thinning amount changes with time, and delays the occurrence of melt in the upper mantle; when the oceanic crust accretion reaches stability, the theoretical oceanic crust thickness produced under the hydrothermal circulation is thicker than that with no hydrothermal circulation. Combined with the analysis of the change of convective heat flux in the process of oceanic crust accretion, the upper mantle temperature in the early stage of oceanic crust accretion is low, the heat source driving the hydrothermal circulation is small, the convective heat flux generated is relatively small and unstable, and the hydrothermal circulation slowly cools the temperature of the top of the upper mantle, thereby delaying the initial melting time of the upper mantle, weakening the melting of the upper mantle, and causing the formation of the theoretical ocean crust thinner than the normal theoretical ocean crust within a certain time period. When the oceanic crust accretion reaches stability, the convective heat flux reaches the maximum and stable. The hydrothermal circulation continues to cool the temperature of the top of the upper mantle continuously and rapidly, resulting in the heat of the deep part of the upper mantle being replenished upward to the top of the mantle, which in turn increases the temperature and melting of the top of the upper mantle, thereby increasing the thickness of the theoretical oceanic crust. With the increase of the expansion rate, the thickness of the theoretical oceanic crust increases, the convective heat flux increases, and the maximum thinning of the phased thinning of the oceanic crust caused by the hydrothermal circulation also increases, and the time of phased thinning is shortened. Combined with the analysis of the structural characteristics of the oceanic crust in the southwest sub- basin of the South China Sea: two seismic profiles across the southwest sub- basin of the South China Sea show that there is an abnormally thin oceanic crust area in the ocean basin, and the thinnest oceanic crust thickness difference between the two seismic profiles is 0. 85 km, and it is speculated that the difference in the thickness of the abnormally thin oceanic crust in the ocean basin and the thinnest oceanic crust at different expansion periods is affected by the phased thinning of the oceanic crust during the hydrothermal cycle during the expansion period.