In trade credit, supplier's credit risk will be increased when retailer's sales ability is his private information. In order to help the supplier screen retailer's sales ability types, an inventive model is built by using the incentive theory, and the incentive contract is obtained by solving it, which is compared with the contracts in trade credit under symmetric information and in cash trade under asymmetric information separately. Results show that, this incentive contract can screen the retailer's type, volumes of transactions are twisted down under the asymmetric information, and the distortion rate and the cost of screening are reduced in trade credit. Finally, a case study verifies the main conclusions.