Abstract: As to concrete-sort materials, whether stresses have been uniformly distributed along the specimen before fracture occurs is a key issue. By using characteristics method of wave propagation, the SHPB tests for cement mortar which is modeled as ZWT visco-elastic material and the elastic material separately are numerically studied in the present paper, to see how the stress is distributed during both loading and unloading. It is found that different rise-times of incident waves evidently influence the stress uniformity, and the optimum rise-time seems to be τs=2tL, either shorter or longer rise-time will result in worse stress uniformity; the stress amplitude has no influence on the stress uniformity, but significantly influences the value of strain and average strain rate in specimens. The error found in the determination of dynamic stress-strain curve of rate-dependent brittle materials with the traditional SHPB techniques of either 3-wave method or 2-wave method, is actually unaccepted, although the dynamic fracture stress can be correctly determined if the requirement of stress uniformity is satisfied before specimen fracture.