In the process of drilling offshore gas hydrate ("hydrate" for short) formations, the invasion of drilling fluid into formations may lead to hydrate decomposition, which will result in borehole instability. In order to solve these difficulties, this paper experimentally studied the decomposition law of methane hydrate under different decomposition modes. Then, the Peng–Robinson equation was introduced to calculate the change of methane gas mole number, and the experimental method for evaluating the inhibition of hydrate decomposition by drilling fluid was optimized. Finally, the effects of the kinetic inhibitor DY-1 and modified lecithin on the decomposition characteristics of hydrate were experimentally analyzed using this method, and the water-based drilling fluid suitable for gas hydrate formations was further optimized. And the following research results were obtained. First, the best way to evaluate the inhibition of drilling fluid on hydrate decomposition is to inject cold solution without relieving the pressure. Second, the DY-1 and modified lecithin can retard hydrate decomposition by adsorbing in hydrate surface to inhibit heat and mass transfer, so they can be used as hydrate decomposition inhibitors of drilling fluid. Third, the optimized drilling fluid system has good low-temperature rheological properties and shale inhibition and lubricity and can inhibit the formation and the decomposition of hydrates, so it can meet the basic requirements of drilling fluid technology for offshore gas hydrate formations. It is concluded that the research results can provide a drilling fluid technical support for the development of hydrates in China.