The present invention relates to an inorganic/organic hybrid composite fire retardation agent and an application of the inorganic/organic hybrid composite fire retardation agent in PET fibers. The preparation method comprises: uniformly stirring and mixing dried PET slices, a phosphorus fire retardation agent and a nanometer material at a temperature of 30-180 DEG C, carrying out extrusion granulation at a temperature of 240-300 DEG C to obtain fire retardation PET slices, drying the fire retardation PET slices, cooling to a temperature of 40-90 DEG C, adding a dispersing agent to obtain a spinning raw material, and carrying out spinning and drafting processing to obtain the finished product. According to the present invention, the PET slices has characteristics of good spinning molding and excellent fiber quality; and the adopted nanometer material and the phosphorus fire retardation agent can produce the synergistic effect, such that the fire retardation agent consumption can be effectively reduced so as to reduce the cost, the enhancement effect can be provided, the mechanical property of fibers can be effectively increased, and the melt dropping resistance of the PET can be improved.