The invention discloses a preparation method of a cyclic phosphonate flame retardant, which relates to the technical field of organic phosphorus flame retardant synthesis. The method comprises the following steps of taking trimethylolpropane and trimethyl phosphite as raw materials with the feed mole ratio of 1: 2-4, selecting methyl benzenesulfonate or methyl p-toluenesulfonate as a catalyst, controlling proper process parameters such as catalyst quantity, temperature, time and the like, and carrying out transesterification and rearrangement reaction to obtain the phosphonate flame retardantwith a cyclic structure and the byproduct dimethyl methylphosphonate flame retardant. The method provided by the invention has the advantages of few raw material varieties, easy source, low cost, rapid reaction, high efficiency, environment-friendly process, energy conservation and consumption reduction; the product prepared by the method is the same as the product prepared by the existing method,but the production period is shortened by 2/3-3/4, the production capacity is greatly improved, and the method is suitable for industrial production.