Improving water productivity (WP) in grain production (WPg) is critical for maintaining food supply in regions with water shortage. A long-term field experiment from 1980 to 2019 was used to investigate the changes in WPg and its influencing factors for summer maize in the North China Plain (NCP). The results showed that grain yield increased from 5.1 t ha(-1) to 8.8 t ha(-1) and WPg from 1.35 kg m(-3) to 2.36 kg m(-3) during the past four decades under full irrigation condition. Overall, a 75% increase in yield and a 73% increase in WPg were achieved. Further analysis indicated that in 1980 s and 1990 s, the improvements in yield and WPg were mainly associated with a significant increase in biomass production which was related to the increase in fertilizer use and the improvement in soil fertility. In 2000 s and 2010 s, the increase in yield and WPg was due to a significant increase in harvest index (HI) which was related to the introduction of modern hybrid maize. Overall, increased HI contributed to 65% of the WPg increase for the past 40 years. The results also indicated that during the recent years, large seasonal yield variation was observed which was related to some deteriorating weather factors such as the reduced sunshine hours and the large variation in temperature. Based on the linear positive correlation of grain yield with WPg, management practices that can minimize the negative effects of weather on grain production would favor an improved WPg in future.