With maintaining gas hydrate stability in the pressure coring tool as research purposes， and in view of the problems with the existing large diameter pressure coring system， such as small core diameter at large drilling diameter， complex drilling tool structure， heavy weight， which makes it not applicable to the actual working condition of the underwater drilling rig， the pressure coring tool was redesigned structurally and indoor pressure tests were carried out. The results showed that the small diameter thin wall pressure-coring tool is reasonable in size and compact in structure， which can meet the requirements of the underwater drilling rig. Laboratory tests showed that when the pressure was maintained below 10MPa， the pressure decreased by 15% within 6 hours； when the pressure was maintained between 10MPa and 30MPa， the pressure dropped by 10% within 6 hours， and the pressure-coring tool can meet the design and service requirements when used within the above pressure ranges.