The paper analyzes the purchasing policy for a supply chain with a retailer and a bank, in which the retailer has financial constraints. By introducing the option procurement and trade credit, the performance improvement and risk reduction are realized. By analyzing the optimal option and fixed order quantities for the retailer and the optimal loan rate for the bank, the explicit expressions and existence conditions are obtained. By comparing the other ways under financial constraint, it is found that trade credit is the most efficient way for the retailer to improve the performance and to realize the global optimum.