Grain weight is the most important factor of yield. Based on the summer maize field moisture control experiments from 2013 to 2015 in Gucheng, the EcologicalMeteorological Integrated Observation Experiment Station, Hebei Province, the natural environment is simulated by multiple scenarios, and the change characteristics of grain weight per spike of summer maize with the development stage (DVS) under watersufficient conditions are studied. As the experiment designed, the water at the three major development stages of summer maize, i.e., emerging to jointing, jointing to tasseling, tasseling to maturity, as well as (emerging to jointing)+(jointing to tasseling), (emerging to jointing)+ tasseling to maturity), and (jointing to tasseling)+(tasseling to maturity), are controlled in turn. By comparing the yield changes between different water treatments, the effects of different durations, different developmental stages and different degrees of water stress on yields are analyzed. The quantitative relationship models between soil relative humidity and yields under different water stresses are established. The results show that under the condition without soil water stress, the kernel weight of summer maize shows a “slowfastslow” growth characteristic with the increase of DVS. During the period from milk ripening to 10 days after milk ripening, the increase range and rate of grain weight are the largest. When water stress occurred from 10 days after tasseling to the stage of milk maturity, the grain weight decreased most. When water stress occurred in the emerging to jointing (stageⅠ), jointing to tasseling (stageⅡ), tasseling to maturity (stage Ⅲ), ［stageⅠ+stageⅡ］ and ［stageⅡ+stage Ⅲ］, the yield of summer maize had a significant linear positive relationship with soil relative humidity in five different periods separately. The relationship model shows that the more severe the water stress and the longer the duration, the greater the reduction in yield is. The effects of water stress on yield at the same level, different stages and different durations are as follows: ［stage II+stage III］ > ［stage I+stage II］ > stage III > stage II > stage I. In the five water treatment scenarios, soil relative humidity went down by 10%, the grain weight reduced 89.7 g/m2、122.7 g/m2、129.8 g/m2、133.7 g/m2 and 144.4 g/m2, respectively, a reduction of 228% to 36.8%.