The rupture of some high-energy coolant pipes in the reactor will cause high-temperature and high-pressure coolant to discharge into the low-pressure area, forming an under-expanded flashing jet. The substantial steam generated by flash evaporation can cause a rapid increase in pressure and may lead to local overpressure. In this paper, a virtual source method to simulate the highly under-expanded flashing jet in a confined vacuum condition is established, considering the effect of the relaxation of flash evaporation and rapid change in back pressure within the confined space on the flash fraction. A three-dimensional analysis model of the Ingress of Coolant Event experiment is established, and the virtual source method is verified with peak pressure and its arrival time. Meanwhile, the three-dimensional spatial distribution of pressure and temperature is achieved. The results indicate that the predictions of peak pressure, peak arrival time and the peak temperature during the discharging process are consistent with the experimental results. Due to jet impingement, a local high-pressure area appears near the vessel wall. The fluid temperature is lower than in other regions due to expansion and flash evaporation. This inhomogeneity gradually weaken as the fluid reaches saturated condition. The proposed virtual source method solves the problem of significant deviation in predicting flash fraction under the scenario of rapid back pressure change in the virtual source theory based on the constant back pressure assumption.