A product inventory optimal policy under impact load is established in this paper. The properties of the policy are that every production period is partitioned into multiphase in which time duration is different , the produc2 tivity can be less than demand rate in every product phase(the maximum productivity is lower than the maximum de2 mand rate) , and preproduction are permitted. Given the bounded productivity , we obtain a product2inventory opti2 mal policy which supplies a theoretic base for management and decision2making of inventory system by using a one2 dimensional search method