In face of he trend of high quality, high efficiency and low profit, auto parts manufacturing enterprises shift the focus of enterprise management to the reduction of costs and the improvement of efficiency by enhancing its internal management. Therefore, taking a car seat manufacturing enterprise as a case study, with the construction model of inventory shortage cost, production cost, storage costs and other factors taken into consideration, an effort has been made to determine the optimal production quantity and production period to achieve the optimal inventory cost. The model and the numerical results show that such two factors as shortage cost and customer requirement are positively related to the optimal inventory quantity and the minimum cost. Therefore, to reduce the inventory cost under the condition of stock shortage, it is necessary to improve the timeliness of enterprise delivery, implementation of logistics outsourcing and information sharing.