Abstract: The invention relates to an organic phosphorous compound flame retardant, their preparation methods and applications. The organic phosphorous compound flame retardants are prepared by drop-wise adding at least one from the soluble salts of di-alkyl hypophosphorous acid or soluble salts of mono-alkyl hypophosphorous acid as well as at least one from the soluble salts of alkyl phosphorous acid or soluble salts of phosphorous acid into the solution of soluble non-halogen salt of II-IV valent metal to react, or drop-wise adding at least one from the di-alkyl hypophosphorous acid or mono-alkyl hypophosphorous acid, and at least one from the alkyl phosphorous acid or phosphorous acid into at least one from the active oxides or hydroxides of II-IV valent metal to react.