Abstract: In order to study the size effect law of mechanical properties of hydraulic asphalt concrete under dynamic load, uniaxial dynamic compressive tests were carried out on hydraulic asphalt concrete samples with a diameter of 100 mm and height-diameter ratios of 0.5, 1.0, 1.5 and 2.0 respectively under the conditions of ambient temperature of 5 ℃ and strain rate of 10⁻⁵/s~10⁻²/s. The strain rate effect and the size effect on the dynamic compressive strength, elastic modulus and failure mode of asphalt concrete are analyzed. The results show that: (1) The greater the strain rate or the smaller the height-diameter ratio of the sample, the more obvious the failure mode of the sample is; (2) With the increase of height-diameter ratio, the compressive strength decreases and the elastic modulus increases; and (3) With the increase of strain rate, the dynamic enhancement factors of compressive strength and elastic modulus increase nonlinearly. Finally, based on the experimental research, considering the synergistic effect of strain rate effect and size effect, and introducing the strain rate influence coefficient, a calculation model of dynamic compressive size effect of hydraulic asphalt concrete is established, and the rationality of the model is verified.