Abstract: A phosphorus-containing compound, a phosphorus-containing flame retardant, and a preparation method thereof are provided. Also provided is a resin composition which comprises the phosphorus-containing flame retardant and an unsaturated bond-containing resin. The resin composition may be used to make various articles, such as a prepreg, a resin film, a resin-coated copper, a laminate or a printed circuit board, and achieves improvement in at least one, more or all properties including resin filling property of prepreg, flame retardancy, alkali resistance, glass transition temperature, thermal dimensional stability (ratio of dimensional change under heat), thermal resistance after moisture absorption, peeling strength, dielectric constant and dissipation factor.