The decomposition process of methane hydrate in pure water and methanol aqueous solution was studied by molecular dynamics simulation. The effects of temperature and pressure on hydrate structure and decomposition rate were discussed. The results show that decreasing pressure and increasing temperature can significantly enhance the decomposition rate of hydrate. After adding a small amount of methanol molecules, bubbles with a diameter of about 2 nm are formed, and the methanol molecules are mainly distributed at the gas-liquid interface, which greatly accelerates the decomposition rate and gas-liquid separation efficiency. The radial distribution function and sequence parameter analysis show that the water molecules of the undecomposed hydrate with ordered ice-like configuration at the 275K temperature condition, evolved gradually into long-range disordered liquid structure in the dynamic relaxation process. It was found that at temperatures above 280K and pressure between 10 atm and 100 atm, the pressure has no significant effect on hydrate decomposition rate, but when the pressure is reduced to 1 atm, the decomposition rate increases sharply. These findings provided a theoretical insight for the industry exploitation of hydrates.