Abstract: Severe oversize distortion frequently occurs in the manufacturing of the core barrel, which affects the processing quality of the core barrel and the construction period of nuclear power plants. This paper analyzes the effecting factors and causes of oversize distortion of the core barrel, and provides the measures to control the distortion, which achieves good results in the following manufacture of the core barrels. Moreover, the improvement and adjustment of the barrel design is provided and applied to the Hualong No.1 project which is the Chinese third generation nuclear power plant.