This study explores the dynamic changes of English writing self-efficacy and writing performance of 330 first-year Non-English Major students in an authentic college English writing course context and probes into the tentative effects of writing self-efficacy on writing performance by means of a cross-lagged design combined with typical case studies. The results reveal that both writing self-efficacy and writing performance of the students changed significantly after a semester of writing instruction and the writing self-efficacy has proven to be a significant source of effect on students’performance. Analyses of the interviews with the typical cases suggest that writing self-efficacy may have contributed to increasing writing performance via boosting students’cognitive engagement, motivation and persistence in face of writing tasks.