The invention discloses a preparation method of an inorganic hybrid flame retardant with high thermal stability. The preparation method comprises the following steps: a), preparing oxidized graphene suspending liquid; b), adding soluble aluminium salt into the oxidized graphene suspending liquid, controlling the concentration of the soluble aluminium salt to be 50-400 mmol / L, uniformly stirring, adding soluble phosphinate at the temperature of 0-80 DEG C, and stirring, so as to obtain reaction liquid, wherein the molar ratio of the soluble aluminium salt to the soluble phosphinate is 1:(3-4); c), enabling the reaction liquid to be subjected to hydrothermal reaction for 3-12 h at the temperature of 170-190 DEG, and cooling, filtering, washing and drying the obtained product, so as to obtain the inorganic hybrid flame retardant with high thermal stability. In addition, the invention also discloses a PBT composite material comprising the flame retardant. The preparation method for the flame retardant, provided by the invention, is simple, mild in reaction condition, high in productivity and low in price, and facilitates large-scale production and control; an experiment result shows that the PBT composite material, prepared by adding the flame retardant into PBT, has the advantages of being high in limited oxygen index, good in anti-dripping effect, high in char forming property and thermal stability, good in heat insulation performance and the like; the comprehensive flame retardancy of the flame retardant is excellent.