Marine landslide due to gas hydrate dissociation is a kind of potential heavy hazards. Numerical simulation was processed to investigate the deformation, landslide and effects of main factors on the critical scale of dissociation zone to induce marine landslides during/after dissociation of gas hydrate. A simple method for analyzing the critical scale was presented based on the limit equilibrium method. It is shown that the maximum settlement is located near the upper side of the dissociation zone. The soil near the lower side of the dissociation zone uplifts. There is a critical scale of dissociation zone over which landslide will occur under given conditions. Expansion modes of the dissociation zone have obvious effects on the critical scale.