The invention discloses a phosphorus-containing fire retardant and its preparation method. A series of the above compound treats diphosphite (which can be a saturated or unsaturated alkyl or aryl ester, and can also be an alkyl or aryl ester containing other hetero atoms comprising N, P, S, Si and the like), formaldehyde and phosphorus oxychloride as raw materials, and is prepared through a two-step reaction. The preparation method has the advantages of mild condition, simple operation and high yield; and the series of the phosphorus-containing fire retardant has the advantages of good heat stability, realization of the halogen-free fire retardation, inhibition of the generation of toxic and corrosive gases in the combustion process, increase of the carbon formation, and improvement of the fire retardation.