Reactor internals are the core components of the reactor pressure vessels in Nuclear Power Plant (NPP), which directly compress, support and position the nuclear fuel assemblies. They are exposed to high temperature, high pressure and high radiation during a long-term service. The fastening bolt may break and fall off due to irradiation stress corrosion crack (IASCC), which directly affects the safe operation of the reactor. Along with the in service time increase, it has gradually become the focus of the in-service supervision of NPP, and it is urgent to perform Nondestructive Examination (NDE) for its structural integrity. In this study, according to the structural characteristics and inspection difficulties of typical bolts, a special ultrasonic combined probe and ultrasonic process are designed, by using simulation technology. The design principles are summarized. In addition, the automatic inspection system of underwater submarine is introduced. The results of the process verification show that currently technology can effectively detect with a minimum flaw sizes of 17.5% cross-sectional area (CSA), which meets the in-service inspection (ISI) requirements of NPP.