In ocean science research, it often requires the integrity of the gas content in samples of seawater. The gas-tight deep-sea water sampler (GTWS) based on pressure self-adaptive equalizer (PSAE) can satisfy the requirement very well. The PSAE is required to achieve a reliable dynamic sealing in deep sea. The general O-ring is difficult to meet this requirement. The polytetrafluoroethylene (PTFE)-coated O-ring is chosen as the sealing parts. The finite element analysis (FEA) software ANSYS is used to analyze the contact pressure of the PTFE-coated O-ring in different conditions, that is, analyze the GTWS’s gas tightness theoretically. Combine with the sea trial results, the feasibility and reliability of the PTFE-coated O-ring in deep-sea dynamic sealing in PSAE can be prove both in theory and practice.