Under the strategy that the supplier provides a delay in payment and a price discount to the retailer, a supplier-Stackelberg game model is established for the problem of determining the trade credit policy. Based on the theoretical analysis and computing the model, the supplier’s optimal conditional trade credit policy, the retailer’s optimal order quality and optimal payment time are presented. The presented numerical experiments show that this conditional trade credit policy can not only stimulate the retailer to make a larger order, but also attract the retailer to pay for his purchases earlier. Hence, it can accelerate the supplier’s capital turnover and realize a win-win outcome.