Abstract: The pressure change caused by marine gas hydrate decomposition during exploitation will result in stress redistribution, sediment creeping and collapse and/or landslides of the upper strata, in addition to casing deformation and wellhead incidents. Taking a well covered marine gas hydrate deposit as an example, we established in this research a mechanical stability model for the upper strata based on the parameters of the upper strata, wellbore pressure and residual formation support force of the gas hydrate deposits when gas hydrate is decomposed, as wellbore pressure change was adopted to exploit gas from the deposits.