Heavy metal contamination in agricultural soils in China is severe, posing a significant threat to the safety of agricultural products. The development of long-lasting and stable in-situ passivation materials has become a current research hotspot for the safe utilization of contaminated farmland. Layered double hydroxides (LDHs) possess unique advantages such as large specific surface area, strong ion-exchange capacity, tunable structure, and super-stable mineralization, offering a new pathway to overcome the limitations of traditional materials. This article systematically reviews the research progress of LDHs in the in-situ passivation of heavy metals in farmland soils from three perspectives: mechanisms of action, material design, and stability evaluation. Firstly, it analyzes the mechanisms by which LDHs synergistically passivate heavy metals through multiple pathways, including isomorphous substitution, ion exchange, adsorption–precipitation, and redox–precipitation. Secondly, it summarizes the effectiveness of both pure LDHs and their composite materials in the in-situ passivation of heavy metals in farmland soils, and discusses the “molecular engineering” design achieved by regulating layer cations, functionalizing interlayer guests, and composite design to enhance targeting capability. Finally, the long-term stability of their passivation effects is evaluated from chemical, physical, and biological perspectives, revealing their potential to resist environmental interference. The article concludes by analyzing current challenges in LDH research and outlining future research directions, aiming to provide insights for the targeted design, precise application, long-term effectiveness, and safety evaluation of LDHs in the in-situ passivation of heavy metals in farmland soils.