Abstract: In the light of the top cap leakage of a pumped storage generator, the total load and residual pre-tightening force on the cap bolt under various working conditions was calculated and it was discovered that the leakage cause was the small residual pre-tightening force of less than 0. Through adjusting the bolt extension, the residual pre-tightening force became larger than 0 and the leakage was improved, but the pre-tightening coefficient still failed to meet the related demand. The serious analysis and re-examination showed that although the pre-tightening safety coefficient could be increased through increasing pre-tightening force for the original bolts, but in the meantime, the total load on the bolt increased, resulting in safety coefficient decease and the possibility increase of bolt broken. Aiming at these problems, the countermeasures were put forward, i.e, increasing the bolt diameter from 36 mm to 52 mm to increase the tightening force and the tightness of bolt connection as well as the ensuring the safety coefficients. The improved bolt serves safely in unit operation.