Poor grain filling of inferior kernel of summer maize is a major challenge restricting grain yield of summer maize, especially under higher plant densities. Much work so far has proved that tillage practices could regulate soil properties and grain yield of maize effectively. In order to explore the effect of tillage practice on grain filling of inferior kernel of summer maize, field experiments were conducted over two consecutive years (2016-2017), and three tillage practices (T1, no tillage; T2, no tillage with subsoiling; T3, deep horizontal rotary tillage) were assessed. The grain filling parameter showed that final grain weight (A), the time reaching the maximum grain filling rate (T-max), the dry matter accumulation under the maximum grain filling rate (W-max), the mean grain filling rate (G(mean)), and the active grain filling period (D) of T2 were usually higher than that of T1 and T3, and significant difference was observed between T2 and T1. Compared with T1, T2 and T3 reduced the soil bulk density and increased the soil porosity, and significantly increased the root dry matter accumulation in the 0-40 cm soil profile by 13.4% and 28.7%, respectively. Soil water content decreased among tillage practices in the order T2 > T1 > T3. However, T2 and T3 increased the water consumption, compared to that of T1. Moreover, T2 and T3 increased post-anthesis photosynthetic capacity, including the photosynthetic rate, leaf area duration, and chlorophyll content, and further improved the post-antheisis and total dry matter accumulation. The enzyme activities of SuSase, StSase, and ADPG-PPase of inferior and superior kernel in T2 and T3 were also higher than that of T1. In conclusion, the present results indicate that subsoiling could promote the grain filling of inferior kernel of summer maize by regulating soil water content, soil water consumption, and photosynthetic capacity.