Abstract: Core make-up tank is a vital component of the reactor safety system. Under accident conditions, dramatic direct contact condensation may occur in the core make-up tank, causing pressure decrease or oscillation, which would be harmful to its safety injection function. To enhance the prediction accuracy of the process, it is necessary to build a special model to calculate the direct contact condensation phenomenon in the core make-up tank, since in the core make-up tank the steam would go through a steam space before it reaches the liquid surface, which is quite different from the mostly studied conditions, where a pipe is often used to guide the steam to the water. In this paper, the jet flow velocity distribution theory was first employed to estimate parameters like the steam velocity profile and the maximum steam velocity at the interface, and then the hypothesis nozzle analysis method was utilized to get the hypothesis nozzle diameter, the effective velocity, the steam penetration length, the effective steam condensation fraction of the jet flow region, etc. Besides, to properly evaluate the heat transfer temperature difference, based on the conversation of energy, a multi-layer model was used to take the temperature stratification effects into account, and the whole liquid region was further divided into two layers, the hot layer and the cold layer. In this way the heat transfer temperature difference was corrected. Considering the effective steam condensation fraction of the jet flow region and natural circulation region near the water surface, a specific condensation heat transfer model was established based on the interface transport model for the jet flow region and the McAdams correlation for natural circulation region. With the help of the effective steam condensation fraction of the jet flow region, the new model could be used to predict the interface heat transfer of liquid and steam during the whole process of core make-up tank safety injection, including the complete condensation period, the partial condensation period and the natural circulation period. Several sets of transient data from two different experiment systems were used to test the new model, with the core make-up tank initial temperature ranges from 21 to 61 ℃, and steam generator pressure from 0.15 to 8.7 MPa. It is found that the experimental data could be well predicted, and a clear improvement compared to the origin model of Relap5 can be observed, which preliminarily demonstrates the validation of the model. This research may be helpful for the accurate prediction of core make-up tank safety injection process and the safety analysis of related accidents.