While researchers have explored the relationship between writing self-regulation and writing self-efficacy in student performance on academic writing tasks, less research has been conducted on the mediating effect of autonomous motivation on self-regulation and self-efficiency. In this study, researchers surveyed 445 elementary school students in China using the Writing Self-Regulation Strategies Scale, the Writing Self-Efficacy Scale, and the Autonomous Writing Motivation Scale. Researchers then compared the results of student responses to the scale items with scores on three written compositions. The results show that (1) writing self-regulation strategy positively predicts writing scores; (2) writing self-regulation strategies not only directly impact students’ writing scores, but also affect students’ performances indirectly through the mediation of writing self-efficacy; and (3) autonomous writing motivation modulates the first half of the “writing self-regulation strategy → writing self-efficacy → writing scores” path. Compared to students with low autonomous writing motivation, the writing self-regulation strategies of students with high autonomous writing motivation are more effective in enhancing their writing self-efficacy.