Most papers about trade credit in supply chain studied retailer's inventory policy based on information shared.Few papers paid attention to supplier's trade credit policy under asymmetric information.So this paper tries to propose supplier's optimal trade credit policy to reveal retailer's private information.The aim is achieved by developing an incentive model with revelation principle.The retailer's private information can be found out through this trade credit policy.This contract is more general than the wholesale price contract.For the retailer's private information,the order quantity and ratio of delay in payment are distorted.Sensitivity analysis shows that the contract is influenced by sales ability and discount rate.Finally,the indirect mechanism with the same effect is proposed to make it easy to be put into practice.