A pressure-retaining sampler was proposed aiming at the problem that it is difficult to obtain high-quality samples by non-isobaric sampling methods. A hollow piston sampling structure was proposed considering the limitation of deployment weight. The non-adaptability of the ideal gas state equation which is used to design accumulators in engineering applications was analyzed for the ultra-high pressure environment in the deep sea, and a design method based on the real gas state was proposed. A liquid active compensation technique was designed for the pressure drop caused by the seal movement. The internal pressure test results show that the pressure vessel and the piston have sufficient strength and good sealing. The high-pressure chamber test shows that the proposed sampling system can work stably under high pressure, and the designed pressure compensation system can significantly improve the pressure retention performance of the sampler. The developed sampler captured 174 amphipods in the Marianas Trench at depths of more than 10 800 m.