Parallel programs can not be debugged as sequential ones repeatly because of their nondetermination，SO tracing and replay become the key technique of parallel debug-ging，and the most overhead of this technique is tracing the events．This paper improves the optimal tracing method provided by Rober Netzer etc．by combining the characteristics of PVM(parallel virtual machine)，makes the events tracing optimal even when non-tran-sitive message race appears．The improved method has been implemented in PVM_based parallel clebugger．