Abstract: Lightweight aggregate concrete (LWAC) is utilized more and more in practical engineering structures because of the light weight and good thermal insulation performance. In this study, LWAC was regarded a three-phase composite consisting of aggregate particles, a mortar matrix and the interface transition zone. A meso-scale simulation method for modelling the dynamic failure of concrete was established. The plastic damage constitutive model coupling with the effect of strain rate was adopted to describe the mechanical properties of concrete meso-components. The compressive failure behavior and size effect of LWAC under dynamic compressive loading were studied. The simulation results indicated that with the increase in strain rate, the inertia effect became dominant, and the size effect on dynamic compressive strength was gradually weakened and suppressed. At the critical strain rate, the size effect would be completely suppressed. In addition, according to the influence mechanism of strain rate effect, the mechanism of size effect on the dynamic compressive strength of LWAC was studied. A semi-empirical and semi-theoretical "static-dynamic unified size effect law" for quantitatively describing the size effect of LWAC was subsequently established.