An optimal inventory model is proposed in this paper. Firstly, the prerequisite of this model is given out. Under the supposed condition of deterioration, the demand rate of articles depends on the inventory level, meanwhile the status of shortage and partial back-order. The optimal inventory model of fixed free back orders is on this basis. With those numerical values in this model, the optimal ordering cycle and ordering batch of stock system are counted out and the sensitivity is analyzed by using the mathematical software Mathmatica 7.0.