The water quality control in PWR secondary system has close relationship with the service life of key important components, such as SG. With the improvement of water chemistry management of secondary system in Qinshan nuclear power plant（QNPP）, the water quality of secondary system becomes better and better. These improvements include updating of management concept, adoption of high AVT of secondary system, optimized operation of condensate polisher and strictly chemical control during outage and startup period. With the implementation of these measurements, the WANO chemistry performance index improved from 1.66 in 1997 to 1.00 in 2006, reaching the advanced level of the world. The improvement of secondary water chemistry ensures the integrity of SG tubes in QNPP.