Abstract: The selection and promotion of mechanically grain-harvested maize varieties pose new challenges to the full utilization of heat in northern spring maize regions. Under the background of climate warming, exploration of the potential of advancing the sowing date can provide a basis for making full use of regional heat resources and obtaining sufficient dehydration time for mechanically grain-harvested maize. In this study, suitable local grain-harvesting maize varieties were used as test materials. Experiments were carried out in six ecological regions of Inner Mongolia, which are the east region of Xing’an Mountain (EXM), south region of Xing’an Mountain (SXM), west Liao River Plain (WLR), north region of Yanshan Mountain (NYM), Tumote Plain (TMP), and Hetao Plain (HTP). The results showed that in the spring maize areas of six ecological regions of Inner Mongolia, maize was sown six to eight days ahead of the conventional sowing date for mechanical grain-harvesting, and the maize yield could be increased by 7.5%–18.4% under the optimal sowing date. The suitable sowing date window of each ecological region was different: EXM from May 4 to 11, SXM from April 27 to May 10, WLR from April 15 to 30, NYM from April 9 to 26, TMP from April 9 to 29, and HTP from April 3 to 22. With an increase in latitude, this period gradually became “narrower”. For every 1° increase in latitude, the average window period was shortened by 1.8 d, and the average advance of the sowing date was reduced by 2.8 d. The sowing date had no significant effect on the corn harvest index. At the “source” end, the development of pre-anthesis leaf area index, pre-anthesis photosynthetic potential of the mechanically grain-harvested maize population were promoted, which significantly increased the population biomass. At the “sink” end, the population sink activity and yield per plant were improved by increasing grains number per ear and 1000-grain weight. In Inner Mongolia, the yield of spring maize mechanically grain-harvested in the security window for suitable early sowing was increased by 10.4% through ascension of “source” development pre-silking, extension of the persistent period of leaf area, optimization of “sink” activities, improvement of the yield per plant and full use of the heat resource condition. Meantime, the mature seed dehydration occurred at an effective accumulated temperature of 43.7 ℃ to 130.9 ℃. This is beneficial for increasing yield and harvest quality of mechanically grain-harvested maize.