過去的存貨理論相關文獻中，有些存貨模式假設存貨供需系統是單階層的，但是在實際的存貨供需系統中，許多情況都是二階層的。另外，過去的存貨理論相關文獻中，大部分的存貨模式總是假設存貨供需系統中僅有單一供應者與單一需求者，但是在真實的存貨供需系統中是存在著多位供應者，因此當需求者的需求量超過某單一供應者所提供的存貨量時，需求者會轉向其他供應商以補足缺額，本文稱此行為是需求者的 “轉向購買”行為。因此本文提出需求者 “轉向購買”之二階層存貨模式並加以測試﹐經測試結果發現若是存貨供需系統中的需求者採取 “轉向採購”的話，則各供應者的存貨水準可降低，而且隨著“轉向採購”程度的提高，整個存貨系統所產生的總利潤就越大。

In the past papers, some of the inventory models always assume the inventory systems are one-echelon. But in the real world the inventory systems are two-echelon. In addition, most of the inventory models proposed in the past papers always assume that there are only a single supplier and a single buyer in the inventory systems. But there are multiple suppliers in the actual inventory systems and the buyer will take the “shift buying” behavior while one supplier can not offer enough inventory quantity. Thus in this paper, we develop a two-echelon inventory model with considering buyer’s “shift buying”. Finally, this model is tested by a simple case and we find that suppliers could decrease the inventory quantity if buyers take “shift buying” behavior, and the higher the “shift buying” rate, the more the total profit in the inventory system.