The exploration and exploitation of natural gas hydrate need to obtain the pressurized core from the formation for physicochemical property testing， reserve evaluation and mining technology research. However， in the process of drilling， gas hydrate is easily decomposed with the change of temperature and pressure， which brings great difficulties to the core drilling of hydrate. In order to solve the pressure maintaining reliability problem of gas hydrate pressure maintaining coring drill， the project team developed a new type of gear and rack closed ball valve for gas hydrate pressure maintaining coring drill. The ball valve was driven by lifting force of rope fishing. The structure and working principle of the pressure maintaining core drill are introduced in detail. The specific pressure of working seal of ball valve and lifting force required for fishing inner pipe are calculated. The indoor test and sea test of the pressure maintaining core drill are introduced. The research shows that the hydrate pressure retaining coring drill has the advantages of simple coring process， good reversing and sealing performance of ball valve， high success rate of coring， and all functions and performance indexes meet the design requirements.