Gas hydrate is a promising energy source because of its great storage under the sea. However, it is not easy to exploit as it becomes unstable and is easy to dissociate when it is disturbed and may trigger a submarine slide and cause a great impact to the safety of human beings and facilities in the sea. In this paper, changes of strength and pore pressure in a soil caused by the dissociation of gas hydrates are discussed. The shear strength of the soil containing gas hydrates decreases significantly with the dissociation of gas hydrates, which reduce the safety factor of a slope but not sufficient to cause failure. During a quick dissociation of gas hydrates in soil, gas or water in pores can hardly escape, and the released gas is greatly compressed and the pore pressure can be calculated. The strength and the pore pressure changes were considered in an infinite slope model. The result shows that only a small amount of gas hydrates has dissociated before the failure of the slope occurs. It is the increasing pore pressure rather than the reduction of soil strength that causes the failure of the slope. Parametric study was also carried out on gradient of slope, water depth and embedment depth of gas hydrates in the seabed. It was found that the gradient of a submarine slope is not an important factor to the failure of the slope while the water depth and the embedment depth affect the stability of the slope significantly.