Based on three-dimensional fast Lagrangian finite difference method of fluid-solid coupling theory of porous medium and elasto-plastic limit equilibrium theory, according to the QK-2 well drilling data of gas hydrate drilling test well in Qiangtang basin, the analysis is made on the mechanism of borehole instability under the condition of seepage and the numerical simulation analysis is also made on the wall stability of instable section to learn stress distribution of surrounding rock around the well and deformation laws of borehole collapse or crack in different drilling fluid density. The safe density window of drilling fluid is determined for no formation collapse (diameter reducing) and no leakage for the gas hydrates drilling test well in Qiangtang basin, so as to provide the evidences for the determination of casing program design and reasonable drilling fluid density.