Existing studies on L2 motivation have established a significant association between ideal self, learning engagement, and academic achievement, but few have examined the internal relationships between these variables in the L2 writing domain. In addition, students' linguistic competence, a fundamental element of writing achievement, remains underrepresented in most research on L2 integrated writing (IW). Considering that learner motivation and engagement may vary across different tasks, this study invited 589 Chinese secondary school students to complete tasks of story continuation (as an IW task) and linguistic competence, as well as a questionnaire relating to ideal writing self and engagement. Structural equation modeling results showed that: (1) linguistic competence was a significant direct predictor not only of students' writing achievement but also of their ideal L2 writing selves; and (2) despite the significant contribution of ideal L2 writing selves to the three kinds of learning engagement, only behavioral engagement was a distinctive and unique mediator that materialized students' inner psychological drive towards writing performance. The pedagogical implications are discussed.