Abstract: Isopropenyl phosphonic acid (IPPA) has been obtained by the reaction of phosphorus trichloride with aceton in the medium of acetic acid. And polyisopropenyl phosphonic acid (PIPPA) was obtained by the polymerization of IPPA initiated with suitable initiator. The results from spinner test, electrochemical corrosion procedure show that homopolymer PIPPA has an excellect property to inhibit the corrosion of mild steel in tap water. And the surface techniques Auger electron spectroscopy(AES), X ray photoelectron spectrocopy (XPS) has been used to research the composition structure and forming mechanism of the protective film on the surface of mild steel coupon. The results show that PIPPA is an excellent agent as corrosion inhibitor in water treatment.