Microplastic pollution has emerged as an environmental issue requiring urgent attention in farmland ecosystems, involving multiple interdisciplinary fields such as soil science, environmental science, plant physiology, and ecotoxicology. This review systematically summarizes the transformation processes of microplastics at different interfaces within farmland soil–crop systems, with a focus on the transformation mechanisms and migration pathways occurring at the soil–liquid, soil–root, and root–stem interfaces. It further outlines the accumulation characteristics of microplastics within plants and their transfer behaviors in the “soil–crop–animal” food chain. The behavior of microplastics in the rhizosphere and within plants exhibits pronounced particle size effects, material-specific properties, and bioregulation features, enabling them to cross plant root barriers and migrate to edible tissues, thereby potentially posing risks to animal and human health via food chain exposure. Future research should strengthen the understanding of interface transformation mechanisms and in-plant migration pathways of microplastics in farmland soil–crop systems, with particular emphasis on their ecotoxicity, bioavailability, and risk thresholds. Such efforts will facilitate comprehensive assessments of food chain exposure and the development of pollution control technologies, providing a scientific basis for agricultural environmental safety and human health protection.