In a 1250-MW pressurized water reactor (PWR), coolant is injected into the reactor vessel under accident conditions through the method of direct injection, which is the most important function of the emergency core cooling system. Since the problem has been found that safety injection start-up will have a significant thermal effect on the reactor’s internal system, a confirmatory study of an improved structure is required in the initial design stage. In this paper, the heat transfer and flow characteristics of the core barrel, the neutron shielding panels, and the radiation surveillance capsules are investigated by a scaled experiment combined with a numerical method to obtain the distribution of the wall temperature and the convective heat transfer coefficient on the outer wall of the reactor internals under different injection conditions. In addition, potentially dangerous parts have been pointed out, and dimensionless correlations are fitted to describe the heat transfer laws of key parts of reactor internals for use in reactor design. This research fills in the gaps in the study of heat transfer under direct injection of the reactor internals in a PWR, providing support for the safety of the reactor structure.