To explore the effect of thoracic impact timing related with the cardiac cycle on blunt cardiac injury，a biomechanics model of the heart was developed and validated. Simulations of thoracic impact with 40 hearts in different phases of the cardiac cycle were conducted. The geometry of heart model was obtained from medical imaging，including the structures of four heart chambers，heart valves and vessels etc. The fluid-structure interaction between heart and blood was considered by the method of surface-based fluid cavities in Abaqus. The different phrases of hearts were presented by the intracardiac pressure and valve activation. The simulation results show that: （1）the established heart model can present the correct impact response of the heart under thoracic blunt from the curves of intracardiac blood pressure in the testing ranges. （2）When the passenger thoracic was under the blunt，the peak of intracardiac blood pressure in the left atrium was （164.91±17.33） kPa，which was obviously higher than that of the right heart，causing that the mitral valve is more vulnerable than the tricuspid valve; the stress on the right heart was（1887.07±168.74） kPa，which was higher than the left heart，so the heart rupture is more likely to occur on the right heart. （3）When the heart was impacted during ventricular filling period, the stress on the myocardium was（1901.3±150.7） kPa, which was higher than that during other impact periods. （4）Initial intracardiac pressure provided little effect on peak pressure, and the Pearson correlation coefficient was less than 0.2; while the state of the atrioventricular valves, open or closed, affects the myocardial injury a lot. This study is helpful to further understand the mechanism of blunt heart injury and provide a basis for vehicle safety design.