Submarine pipeline gas releases and dispersions can cause safety concerns such as fire and explosion, which can cause serious casualties and property losses. There are many existing studies on the impacts of the horizontal diffusion distances of natural gas leakages from subsea pipelines, but there is a lack of research on the impact of influencing factors on vertical diffusion distances. Therefore, a diffusion model of natural gas leakage from a submarine pipeline is established by using the computational fluid dynamics method (CFD). The influence law and degrees of factors such as water depth at the leakage point, leak orifice size, leak pressure and the ocean current’s velocity on the leakages and vertical diffusion distances of natural gases from submarine pipelines are systematically investigated. The results show that the leaked natural gas jet enters the sea water to form an air mass, which rises continuously under the action of the pressure in the pipe and the buoyancy of the sea water. The gas mass breaks into smaller bubbles affected by the interaction between the gas−liquid two phases and continues to float up and diffuse to the overflow surface. It is also found that the ocean current’s velocity will affect the offset of leakage gas along the current direction; the depth of the leakage water, the pressure in the pipe and the leakage aperture will affect the time when the gas reaches the sea surface and the release area after a submarine pipeline’s leakage. The research results would help to support risk assessments and response planning of potential subsea gas release accidents.