In-Vessel Retention (IVR) strategy is a widely adopted severe accident mitigation measure in third generation nuclear plants which conducts decay heat by flooding RPV. However, due to the presence of heat focusing effect, the IVR strategy in high-power reactors is at risk of insufficient safety margins, thus necessitating the application of new high-efficiency heat transfer technologies. Spray cooling is an enhanced heat transfer method achieved through nozzles, which boasts extremely high heat transfer coefficient. Thus, spray cooling has potential capacity to be used in IVR safety enhancement. To translate the theoretical scheme into engineering practice, it is essential to explore the cooling limit and related heat transfer mechanisms of spray cooling under engineering conditions. CNPE has independently designed and constructed Spray-cooling Mechanism for Improvement of heat transfer Limit Experiments (SMILE) apparatus to study spray cooling heat transfer performance. This SMILE apparatus mainly consists of the experimental section, the circulating loop system, and the data measurement and acquisition system (DAS). The experimental section can be further segmented into the spray chamber, the heating device, and the corresponding nozzles, constituting the core part of the bench. The circulating loop system encompasses water tanks, pumps, pipes, and corresponding valves, which are utilized to provide the corresponding operating conditions for the experiment. The data acquisition system collects the temperature, pressure, and flow signals within the bench and gathers visual materials. The heating device is designed with a maximum heating area of 20 cm × 20 cm and a maximum heat flux density of 3 MW/m2, attaining uniform heating through the electric heating film and copper block. Three layers of thermocouples are arranged inside the copper block to measure the temperature gradients at different positions which are used to calculate the heat flux. The spray chamber contains an internal nozzle support device, an external rotation device, and related interfaces. The QGA series nozzles of Spraying SYSTEM Co. are installed through the support plate, enabling the 2 × 2 array distribution with different distance between each. Pre-experiments were conducted by single nozzle, and the results showed that the Sauter Mean diameters of different nozzles is similar. It gradually decreases with increasing pressure and increases with larger capacity size, while the temperature of the heated surface monotonically rises with higher heat flux. This bench is capable of conducting research on the spray cooling heat transfer limit and the heat transfer coefficient and can also investigate the impacts of multiple factors on the heat transfer capacity, comprehensively exploring the key issues regarding the application of spray cooling in the IVR measure.