The small break loss of coolant accident (SBLOCA) is a typical accident in the nuclear power plant, and the diagnosis of accident parameters in a small break loss of coolant accident (SBLOCA) is a very important research direction in the field of nuclear safety. The timely and accurate judgment of the break parameters can help the operator understand the serious situation of the SBLOCA and avoid more serious consequences. In this study, a physical model of the blowdown process during SBLOCA was established by using the theory of two-phase critical flow, flash evaporation, and energy conversion. Then a diagnostic model for the break diameter of SBLOCA was established by combining the NSGA-II and the physical model, and the experimental results obtained by the blowdown experiment of simulated pressurizer under a low pressure condition were used to verify the accuracy of results obtained by the diagnostic model. The results show that the diagnosis model can be used to obtain the break diameter during the blowdown process of SBLOCA. In different conditions of the blowdown process, the errors between the diagnosis results and the experiment results do not exceed 20%. The calculation time of the diagnostic model is less than 400s.