Based on Hughes’s PPP Washback Effect Model (1993) and Alderson & Wall’s washback effect hypothesis (1993), this study explores the washback effect of interpreting tests. It takes the China Accreditation Test for Translators and Interpreters (CATTI) interpreting test as an example, draws on examination, questionnaire survey, and in-depth interviews, and investigates the effects of the test on the learning process and outcome of 139 undergraduate or graduate candidates. Results show that the test has an overt and positive washback effect on the learning process and outcomes. In terms of learning process, the test has a great impact on the content, methods, and attitudes in interpreting learning; in terms of learning outcomes, the test has greatly increased the motivation of interpreting trainees and improved their competence. Suggestions for further improving the positive washback effect of the test are provided.