With the widely application of High Level Architecture (HLA) standard that supports interoperability and reusability, correct understanding of time management and advance mechanism in HLA has been a concerning difficult problem in the field of distributed simulation. Aiming at the problem, the paper analyzed and compared difference between conservative time advance mechanism and optimistic time advance mechanism. The rollback manager was introduced to implement the optimistic time advance mechanism. The middleware was also applied to deal with the rollback process in simulation model. In order to make fully use of advantages of optimistic synchronization mechanism, Flash Queue Request (FQR) service that provided by HLA was used to arrive at an improved time advance algorithm.