The first purpose of this empirical study was to assess and validate the Academic Writing Self-Efficacy Belief Questionnaire (AWSEBQ) framed by social cognitive theory. The second purpose was to evaluate the predictive effects of different aspects of self-efficacy beliefs on academic writing performance. Data were collected from 743 learners at a Chinese university. The assessment and validation process involved a series of rigorous confirmatory factor analyses. The results validated the hypothesized five-dimensional structure of academic writing self-efficacy beliefs, including linguistic knowledge efficacy (LKE), self-regulatory efficacy (SRE), information organization efficacy (IOE), writing performance efficacy (WPE), and rehearsal and memory efficacy (RME). Model comparisons confirmed the function of self-efficacy beliefs as a multidimensional construct, in which the five factors were intercorrelated. The results of regression analysis demonstrated the significant predictive effects of five dimensions (i.e., LKE, SRE, IOE, WPE, and RME) on English as a foreign language (EFL) academic writing performance. The findings suggest that self-efficacy beliefs can explain EFL academic writing performance. This study ends by providing theoretical and empirical implications for writing assessment and the possible enhancement of EFL academic writing.