Abstract: In order to study the impact fracture of brittle materials, this paper studies the dynamic fracture characteristics of rock Mott ring in the process of free expansion based on the self-developed continuous- discontinuous code. The influence of mesh size on the simulation results is discussed and the accuracy and reliability of the simulation results are verified by mesh refinement and convergence analysis. A new theoretical model of Mott ring dynamic fragmentation based on the strict balance of initial kinetic energy, residual kinetic energy and fracture energy is proposed, and the influence of kinetic energy ratio on the normalized fragment size is discussed. The simulation of rock Mott ring under different fracture energy, tensile strength and elastic modulus was carried out to study the influence of material parameters on fragment size, strain rate and normalization effect. The results show that the new fragmentation model based on energy balance can better predict the average size of fragments and overcome the shortcomings of traditional Grady model and Glenn-Chudnovsky model. The cumulative distribution curve of fragments can be approximately characterized by Rayleigh distribution curve. With the increase of fracture energy, the degree of fragmentation of rock Mott ring decreases under the condition of constant strain rate. At a constant initial expansion rate, the rock Mott ring is affected by many factors such as the cumulative stress level before fracture, the accumulation of strain energy and the stress concentration during the fracture process. The average fragment size does not show a monotonous trend with the increase of tensile strength. The elastic modulus has an obvious threshold for the strain rate effect of the rock Mott ring. When the strain rate exceeds the threshold, the time required for the rock material to move from the elastic deformation stage to the final fracture is smaller, resulting in the effect of the elastic modulus on the average size of the fragment is almost negligible.