Abstract: For a certain type of reactor main pump casing, the bolt holes on the main flange must retain machining allowances before the hydrostatic test, and the final precision machining is performed only after the test is qualified. This paper analyzes the stress distribution of the bolt holes, combining theoretical calculations and finite element analysis （FEA）, and concludes that the strength of the main flange threads does not meet operational requirements. A dedicated test mock-up simulating the load-bearing conditions of the main flange bolt holes was fabricated. Experimental validation confirmed that although thread deformation occurs, it does not hinder the assembly and disassembly of the bolted joint. This finding underscores the importance of integrating theoretical analysis with practical verification.