The present study explored and compared the washback mechanisms of CET-6, IELTS and TOEFL writing modules. The results showed that, in general, achievement test use influenced language learning and test-oriented preparation, mediated by test value; instrument test use directly influenced language learning; test design influenced language learning via test value, and influenced test-oriented preparation via both value and expectancy. The washback mechanisms of the three writing modules differed significantly with regard to the influences of instrument test use on language learning, test design on value and test value on test-oriented preparation. This paper corroborates the specificity of washback in writing modules and the variations of washback among different tests.