Abstract: The methane leakage in deep seabed and its environmental affection assessment is one of the important problems considered in the development of natural gas hydrate. After entering into seawater, the methane moves upward due to buoyancy and forms gas-liquid-solid multiphase flow, accompanied by physical and chemical processes such as hydrate shell formation, hydrate decomposition, methane dissolution and microbial anaerobic oxidation. The decomposition and phase transition rate of pure hydrate under convective heat transfer is a key parameter to evaluate methane migration in seawater. In this article, a new device for hydrate particles decomposition phase change is accomplished which can control the temperature and velocity of flowing water, then the decomposition process of different hydrate particle sizes under different water temperatures and flowing velocities are recorded, formulating the mechanism of decomposition of hydrate affected by convection heat transfer. Based on experiments the real time decomposition rate of hydrate particles and its variation law is analyzed and by dimensional analysis method key variables determining the decomposition time are deduced, then the general dimensionless expression of the hydrate decomposition time under convective heat transfer is obtained by mathematical fitting. The results show that, convective heat transfer accelerates the decomposition phase transition and the decomposition phase transition time decays exponentially with Reynolds number, as power function with temperature difference and the power index is greater than 1.0, then the front of decomposition phase change tends to advance steadily with the enhancement of convection. The results can provide some basic parameters for the environmental assessment of methane leakage and the feasibility argumentation of new mining schemes.