Inadequate writing skills can prevent learners from improving their writing performance and interfere with their subsequent writing performance in authentic scenarios. The article’s research focuses on the effects of metacognitive regulation on students’ authentic writing performance in a web-based constructivist learning environment, which relies on constructivist learning environments to better present the authentic writing problems learners face in their studies and lives. In this paper, we adopt the method of randomized group sampling to conduct a single-group pre-test and post-test experiment on 40 students in a public high school. It also chooses students’ writing learning achievement as the dependent variable, and students’ metacognitive regulation level and writing selfefficacy as the independent variable and mediator variable, respectively, and explores the degree of influence of metacognitive regulation level on students’ writing learning achievement through multiple linear regression. The results showed that there was no significant difference between pretest 1 and pretest 2, while posttest 1 and posttest 2 were much higher than pretest 1 and pretest 2. There was a significant positive effect of students’ level of metacognitive regulation on students’ learning achievement in writing (0.459), and there was a significant mediating effect of students’ writing selfefficacy between students’ level of metacognitive regulation and students’ learning achievement in writing. Relying on the web-based constructivist learning environment can significantly enhance students’ metacognitive regulation level and provide a new teaching path to promote students’ writing learning achievement.