Abstract: The methane leakage in deep seabed and its environmental affection assessment is one of the important problems considered in the exploitation of natural gas hydrate in the world. After entering into seawater, the methane moves upward due to buoyancy and forms gas-liquid-solid multiphase flow, accompanied by complicated physical and chemical processes such as hydrate shell formation, hydrate decomposition, methane dissolution and microbial anaerobic oxidation, and so on. The decomposition and phase transition rate of pure hydrate under condition of convective heat transfer is a key parameter to evaluate methane migration in seawater. In this article, a new device for hydrate particles decomposition phase transition is designed and 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 noticeably accelerates the decomposition phase transition and the decomposition phase transition time decays exponentially with Reynolds number, in the meanwhile decays with temperature difference as power function in which the power index is greater than 1.0. By analyzing the real time decomposition rate of the hydrate spherical particles, it is find that the front of the decomposition phase transition has a steady advancing trend with the enhancement of convective heat transfer. It is expected that the results of this study can provide basic parameters for the environmental assessment of methane leakage in deep seabed and the feasibility demonstration of new exploitation schemes.