Abstract: The bolt of the high pressure inner cylinder of turbine, as a high-temperature fastening bolt, is an important component of the nuclear power plant thermal power equipment. The bolt is prone to fatigue crack in the long run, due to high temperature and high pressure role of bolt material. This article describes the small-angle longitudinal wave angle probe ultrasonic testing of the internal fatigue crack in the threaded portion of the bolt of the high pressure inner cylinder of turbine. Through theoretical analysis and experimental verification, the testing method improves the ultrasonic testing sensitivity about the fatigue cracks.of the root of tooth and inner of the bolt, thus leading to effective detection of small fatigue cracks.of the root of tooth and inner of the bolt.