Polyurethane/magnesium hydroxide (PU/MH) composites were prepared by modifying magnesium hydroxide (MH) with titanate coupling agent. The mechanical properties and combustion behavior of the composites were studied. The results show that with the increase of MH content, the compressive strength of the composites increases gradually, and the tensile strength and bending deformation of the composites decrease gradually. With the increase of MH content, the ignition time (TTI) and mass residual rate (MLR) of the composites increase gradually, the total heat release (THR), total smoke release (TSR), CO generation and CO₂ generation of the composites decrease gradually, and the combustion grade of the composites increases from V-2 to V-0. When the MH content is 30%, the mechanical properties and flame retardancy of 4# composite are good. Compared with pure PU, the compressive strength of 4# composite is increased by 45.0%, the tensile strength and bending deformation are decreased by 10.0% and 5.3%, respectively, the combustion grade increases from V-2 to V-0, and the total CO and CO₂ production are decreased by 50.0% and 22.2% respectively.