Abstract: Five sets of coal samples with a diameter of 75 mm and length-diameter ratios of 0.20, 0.27, 0.33, 0.40 and 0.47 are subjected to dynamic shear tests under varying impact air pressures with a Φ50 mm-split Hopkinson press bar (SHPB) experimental system. The impact air pressures on the dynamic shear strength of coal samples and the length-diameter ratio effects on the dynamic shear strength and loading rate of coal samples are investigated, and a theoretical model is established for the length-diameter ratio effects. The experimental results demonstrate that: (1) The dynamic shear stress time curve of the coal samples can be divided into four stages: initial rise, linear increase, slow rise and fall in stress. (2) The dynamic shear strength and loading rate of coal samples are positively and linearly correlated with the impact air pressures, but there are differences in the magnitude of increase for different length-diameter ratios. As shown by the fact that the smaller the length-diameter ratio of the coal samples, the greater the increase in dynamic shear strength and loading rate at the same impact air pressure increment. (3) The dynamic shear strength and loading rate of the coal samples are both related to the length-diameter ratio, and exhibit positive and negative length-diameter ratio effects at lower ones of 0.25 and 0.35 MPa and higher ones of 0.45 and 0.55 MPa, respectively, and the impact pressures with the least effects of the length-diameter ratio are determined by ANOVA to be 0.376 MPa. (4) A theoretical model for the effects of dynamic shear strength length-diameter ratio on the coal samples under different impact air pressuress is established, and a theoretical model for the effects of loading rate length-diameter ratio is derived from the loading rate effects, and the reasonableness and accuracy of the proposed model are verified.