Abstract: Due to the large diameter and high cost,the traditional random sampling would be a costly method for mechanical property inspection of main bolts of nuclear reactor pressure vessel,which were different from other steel bars.In nuclear power engineering,the hardness deviation method was applied in selecting specimen batch of main bolts,which properly solved the cost problem on the premise of ensuring the inspection reliability.The reasonability and necessity of the hardness deviation method were expounded from the perspective of engineering principle.