The present invention relates to a phosphorus-containing polyphenylene oxide resin, its preparation method, a method for preparing the prepolymer of the phosphorus-containing polyphenylene oxide, a resin composition and an article thereof, wherein the phosphorus-containing polyphenylene oxide resin has a chemical structure represented by the following formula (I): wherein R′ is R″ is R′″ is hydrogen, Through the use of the above phosphorus-containing polyphenylene oxide resin, an article made from the resin composition can has good flame retardance, good thermal resistance and a lower percent of thermal expansion while dielectric properties can be maintained, such that the present invention is suitable for use in products such as copper clad laminate and printed circuit board.