In order to analyze the validity of testing results for CCCD-SHPB dynamic fracture test system, 3-D dynamic finite element method was employed to investigate the effects of specimen’s geometric size and incident/transmitted bar’s material on the testing results. The analysis confirms that the specimen diameter has a significant effect on the testing result of CCCD-SHPB（central cracked circular disk-split Hopkinson pressure bar）dynamic fracture test system, and whether or not matching of the incident/transmitted bar’s impedence with the specimen’s impedence has an effect on the precision of the testing result, but the specimen thickness, to some extent, has less effect on the testing result. Due to the unbalance of loads on two sides of specimen at the early stages of dynamic loading, a characteristic time ΔT, which is dependent on specimen size, is introduced in the present paper. Its value is approximately equal to the time of which stress wave travels back and forth in the specimen for two times. It is necessary that the time of which specimen is loaded till fracture must be greater than ΔT when the dynamic fracture test is conducted.
