Abstract: Sea bottom gas hydrate (GH) may be formed by porewater advecting upward from deep. In order to understand this process, we developed a non-dimensional mathematical model, combining together the sedimentary process, methane transporting by convection and diffusion of fluid upward, and methane solubility, for study of GH formation and accumulation in a temporal and spatial framework. The model describes the process of GH formation and accumulation with 3 dimensionless parameters, Pe₁, Pe₂, \widetilde C_m, ext^__\rml, which represents respectively the sedimentary process, porewater advection upward from deep and methane content in the fluid. GH emerges in the upper gas hydrate stability zone (GHSZ) first, then grows downward within the continuous sedimentary deposits, and extends eventually to the base of GHSZ. There is a negative correlation between GH evolution time and the three parameters of Pe₁, Pe₂, \widetilde C_m, ext^__\rml, and between GH concentration and Pe₁, \widetilde C_m, ext^__\rml, but a positive correlation between GH concentration, evolution time and upward methane flux (Pe₂ and \widetilde C_m, ext^__\rml). The methane solubility influences greatly on the GH formation and distribution. But the simulation results suggest that both the methane concentration of fluid flow from deep and its flux control the methane inputs and outputs of the hydrate system. They are not included in the solubility-curve. So we propose the methane concentration of porewater upward and its flux as controlling factors.