The commonly used halogenated flame retardants in polyurethane have the problem of releasing corrosive gases and toxic and harmful substances such as hydrogen halide during combustion, which poses a threat to human health. In recent years, halogen-free flame retardants have gradually replaced halogen-free flame retardants for flame retardancy of rigid polyurethane foam with the advantages of halogen-free, low smoke and environmental protection. In this paper, halogen-free flame retardants for rigid polyurethane foam were subdivided in two major directions: additive flame retardants and reactive flame retardants, and in the following sub categories: organic, inorganic, intumescent, nitrogen, phosphorus, silicon, nano, etc. The characteristics, flame retardant mechanism, common flame retardants and flame retardant effects of halogen-free flame retardants for rigid polyurethane foam were summarized. The common universal flame retardant mechanisms of halogen-free flame retardants for polyurethane foam were described. At last, the development hotspots of halogen-free flame retardants for polyurethane foam were prospected.