Abstract: Natural gas hydrate mainly occurs in the seabed sediments under low temperature and high pressure. When temperature or pressure changes, the stability of gas hydrate will be damaged, and geological disasters, such as submarine landslide and sediment collapse triggered, which will cause severe damage to offshore platforms and submarine cables. In this paper, based on the seismic data acquired from the submarine continental slope in northern South China Sea, actual reservoir property parameters and corresponding mechanical model parameters are acquired with the improved geomechanical model. Effective formation stress is calculated with the equilibrium equation of pore pressure. Upon the basis, the numerical simulation of submarine landslide induced by the decomposition of natural gas hydrate is made. Then, using the strength reduction method, the mechanical response of hydrate reservoir change caused by initial hydrate decomposition amount and total hydrate decomposition amount is calculated. Upon the correspondent safety factor distribution diagrams, we made a preliminary analysis of undersea landslide caused by hydrate decomposition. The results provide a guidance for prevention of primary or secondary environmental geohazards in the future exploitation of gas hydrate.