The embryo's initial free expansion within a seed is eventually restricted by the testa, which leads to the formation of folds and influences seed maturation through mechanical forces, though the exact mechanisms are not well understood. We manipulated these mechanical constraints on oilseed rape embryos, employing techniques like magnetic resonance imaging, computer graphic reconstruction, and various omic analyses. Our findings indicate that mechanical constraints limit testa and endosperm expansion, causing embryo deformation and accelerating maturation by halting cell proliferation and boosting lipid and protein storage. This process involves changes in cell cycle control, reactive oxygen species metabolism, transcriptional reprogramming, and glycolytic flux regulation. Thus, mechanical constraints play a critical role in prompting embryo maturation.