Pressure coring is considered as one of the key techniques for the natural gas hydrate-bearing sediment coring. To overcome the problem of low successful rate of pressure coring for a pressure corer, an ice-valve-based pressure corer made out of in situ drilling fluid during coring was proposed. A series of experimental tests were conducted by using a specially designed experimental system. The formation process of the drilling fluid ice valves was studied by both experimental tests and computational fluid dynamics (CFD) simulation method. The pressure sustaining capacity of the ice valves was also measured experimentally. Results showed that the sustained pressure of the ice valve increased almost linearly with the increase of the ice valve length. However, the increasing length of the ice valve caused the extension of formation time of the ice valve. To save the coring time, the ice valve length for pressure coring should be as short as possible while satisfying the sealing requirement.