This invention relates to an atom transfer radical polymerization initiating system and its application in the preparation of polymers. The technical problem it needs to solve is to provide an initiating system with less catalyst, cost and environmental pollution. The method is characterized with the general formula [Hmim][RCOO]/PX/hydroquinone/ MX1n, in which, [Hmim][RCOO] is methylimidazol organic acid salt ion liquid and R is alkyl, PX is a initiator with R1 of alkyl and X of halogen, catalyst MX1n is tall-price transition metal mineral salt, hydroquinone is a reductor, and oxygen has no effect on the polymerization system. The polymerization polymer of a monomer prepared from reaction of the above system and the monomers can react with different shapes of initiator obtaining linear, stellate and other polymers.