The invention relates to a preparing method of an aminotris(methylenephosphonic acid) metal salt flame retardant. An aminotris(methylenephosphonic acid) metal salt is prepared by reacting aminotris(methylenephosphonic acid) with a metal chloride. The method includes steps of adding the aminotris(methylenephosphonic acid) and the metal chloride according to a mole ratio of 1:1 into a reaction container provided with stirring, then adding water the using amount of which is 20-50 times of the using amount of the metal chloride, fully stirring the mixture, adjusting the pH value to 1-3, heating the mixture to 100-130 DEG C, reacting the mixture for 24-28 h, and then subjecting a product to filtration, water washing and drying to obtain the flame retardant. The aminotris(methylenephosphonic acid) metal salt has characteristics of easily available raw materials, a low price, a simple and convenient synthetic process, and the like. The prepared aminotris(methylenephosphonic acid) metal salt has advantages of good thermal stability, a high flame retardant efficiency, a wide application range, and the like, and has a wide market prospect.