With respect to a supply chain system which consists of a supplier, a retailer and a commercial bank, when the retailer encounters capital constraint, the loan model of a credit guarantee provided by the supplier under bank's downside risk control can alleviate the impact to supply chain profits. We analyze the optimal order decision of the retailer and the optimal wholesale decision of the supplier in the setting of bank's risk-neutral and bank's downside risk control through designing the retailer's and the supplier's expected profit model respectively, and solve them by Stackelberg game. Finally, the conclusions are verified by sensitivity analysis of the parameters. The results show that: 1) if the guarantee coefficient is given, only when the risk tolerance coefficient is lower than a threshold, the bank's downside risk control has an influence on the retailer's and the supplier's optimal decisions; 2) if the risk tolerance coefficient is given, only when the guarantee coefficient is lower than a threshold, the supplier's guarantee coefficient has an influence on the retailer's and the supplier's optimal decisions; 3) the bank's loan limit is a strict increment function of guarantee coefficient and risk tolerance coefficient.