Three used most commonly specimens：Normal Brazilian disc，flattened Brazilian disc and holed Brazilian disc were simulated by the finite element analysis software LS-DYNA, based on which we adopted flattened Brazilian disc to carry on dynamic splitting tests with granite specimens. By comparing the simulation and test results, we gain the relation of strain rates and tensile strengths of rocks and discuss the optimal specimen size of rock dynamic splitting tests. The results show: Fattened Brazilian disc fits the condition of broking from center well so that the test results will be more credible; For the rock mode used in the paper, we suggest the central angle of flattened Brazil disk to be 20°~24°; If holed Brazilian disc is used in the dynamic splitting tests, the suggested aperture ratio is about 0.4~0.5; Otherwise, the dynamic splitting strengths of rocks are increasing with the increase of strain rate, of which the increase rate decreases while this growing trend is affected by the specimen size observably.