Abstract: Based on the Monte Carlo homogenization theory and the finite volume method, a three-dimensional diffusion spatiotemporal dynamics model suitable for the analysis of instantaneous critical accidents was established. The three-dimensional diffusion spatiotemporal dynamics model was coupled with the unsteady state heat transfer model and the radiation cracking bubble model, and the calculation program GETAC-S was upgraded, so that GETAC-S had the ability to analyze the transient state of the solution system under any geometric and material conditions. GETAC-S was verified by the experimental data of TRACY, which was an international transient device, and the results are in good agreement. GETAC-S was used to invert the process of JCO criticality accident in Japan, and the results show that the GETAC-S has the ability to evaluate and retrieve the consequences of critical accidents in complex solution system, which provide theoretical support for the prevention, evaluation and shielding of nuclear critical accidents.