To optimize the length-diameter ratio of the impact test specimen under three-dimensional static combination conditions,five mixed granite specimens with different length-diameter ratios(diameter 50 mm,length 30 mm,40 mm,50 mm,60 mm and 70 mm,respectively) under constant axial compression(4 MPa) and confining pressure(4 MPa) were test by a modified split Hopkinson pressure bar(SHPB).The influence of aspect ratio on stress equilibrium time and peak strength was analyzed from the aspects of stress equilibrium state,the relative value of the stress difference,and the dynamic stress characteristics.At the same time,a secant modulus is introduced to better illustrate the plastic deformation of the specimen under combined loading.The results indicate that the stress balance of the specimen changes in real-time during the test process,and it is in relative balanced in the period from the starting point of stress quasi-elasticity increase to the peak point of stress.The equilibrium time varies with the aspect ratio,and the equilibrium duration is 1.0>0.6>0.8>1.2>1.4.With the increase in the length-diameter ratio,the number of stress wave reflection required by the relative value of stress difference between the two ends of the specimen satisfying the uniform state increases,and the peak strength fluctuates.The secant modulus of the peak point increases gradually,and an obvious step occurswhen the length-diameter ratio increases from 1.2 to 1.4.It can be seen that the best length-diameter ratio of SHPB test specimens under the condition of low level(≤10 MPa) constant axial compression and confining pressure combined with static load is 1.0.The impact test results are different from those without combined static load servo.It can provide a reference for the optimization of length-diameter ratio of other rock species under similar conditions.