Polyurethane foam, as one of the most important and widely used synthetic polymers since the 20th century, exhibits advantages such as low density, high strength, and excellent thermal insulation. It finds widespread applications in fields like aerospace noise reduction, railway track stabilization, and building insulation. However, its high flammability poses a serious threat to human life and property, limiting its further development. The addition or surface coating of flame retardants can indeed enhance the flame retardancy of polyurethane foam efficiently. However, these methods often result in the migration and precipitation of flame retardants, ultimately compromising its flame retardant properties and internal performance. Alternatively, the copolymerization of reactive flame retardant molecules into the polymer chain offers a more effective solution. This approach not only addresses the issues of flame retardant migration and precipitation but also minimizes the impact on the substrate's properties. As a result, it is highly favored by the industry and holds immense potential for future engineering applications. This review aims to provide a comprehensive overview of various methods for the preparation of intrinsic flame-retardant polyurethane foams over the past decade, focusing on the perspective of monomer molecular design and synthesis. This includes polyol modifications, isocyanate modifications, additive modifications, and other modifications. Furthermore, the review will delve into the challenges that remain to be addressed and offer insights into potential future directions for application development.