Abstract: C.B.Roellgen polymorphic cipher(PMC) has been found wide application in encrypting diskette, but it is not suitable for complicated network environments.For this reason, PMC was improved and a new construction method of S-box was proposed based on a strong one-way function which could satisfy the Strict Avalanche Criterion(SAC).It could create function which could satisfy SAC quickly and conveniently. Finally, the security of the method was analyzed and verified in terms of SAC.The experimental results show that many long periodic sequences which can satisfy cryptography characteristics can be produced quickly and largely by the improved PMC and used as the session keys to both communication parties in network.