This study examines how the revised College English Test Band 4 (CET-4) influenced Chinese non-English major undergraduates' out-of-class English learning practices over time, by following two informants in one university from the day they began their college English learning to the day they knew their results in the target examination. In total, 96 diary entries and 20 post-diary interview recordings were collected in the study. The data analysis shows that the washback effects from the CET-4 on the two informants' out-of-class English learning practices seemed to vary across time, and tended to increase as the target examination approached. Furthermore, such dynamic washback effects appeared to be mediated by the two informants' possible selves concerning the CET-4. The study identified the dynamic nature of washback on individual learners and interpreted the underlying mechanism by gaining insights from possible selftheories.