Requiring highly to create the safe and comfortable environments in the marine systems, the phenomena on the functional design of atmosphere control system in enclosed spaces have to be clarified by an accurate analysis. For settling this problem, it is necessary to establish a suitable method on the turbulent diffusion state of heat and atmospheric gas in which it has the tendency to form thermal layers and heat / density pockets in the absence of properly balanced heat and gas concentration based on turbulent gas flow. A numerical method is presented in this paper to give a precise prediction on turbulent heat diffusion of high Rayleigh number which is governed almost by natural convection. This method is a control volume method in which the SIMPLE procedure is applied and the turbulent flow is solved by the k-E two equations model. The natural convection and heat transfer in an enclosed space is calculated to validate the present method and the effect of the low Reynolds number is also investigated. Furthermore, the characteristic analysis of two-linked cabins with one opening is executed to examine the applicability for marine fire spread phenomenon.