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The development of multi-cropping systems is hampered by the seasonal shortage problem of growing two or three crops within a year. Exploring strategies to alleviate phenological constraints in multi-cropping systems is crucial for increasing grain production. Using a county-level panel dataset with detailed crop progress information from China, this study investigates whether mechanized winter wheat harvest can alleviate the phenological constraints of a shorter growing season for subsequent summer corn in a wheat-corn double cropping system. The results show that mechanized winter wheat harvest considerably extends the length of the summer corn growing season. This spillover effect of mechanized winter wheat harvest is particularly evident in plains and hilly areas with larger farm sizes. Further analysis of the mechanism indicates that mechanized winter wheat harvest affects the length of the summer corn growing season by advancing the planting date and delaying the maturity date. These results underscore the importance of advancing agricultural mechanization to enhance food security under limited land resources.