This paper harnesses collaborative annotations by students as learning feedback on online formative assessments to improve the learning achievements of students. Through the developed Web platform, students can conduct formative assessments, collaboratively annotate, and review historical records in a convenient way, while teachers can generate formative assessments, manage collaborative annotations, and spot blockades to student learning in an efficient way. The experiment spanned one semester and underwent midterm and final exams. The results showed that adopting collaborative annotations on formative assessments significantly enhanced the learning achievements of students and extended retention time in the proposed system. Through a questionnaire and interviews, most students expressed positive attitudes toward using the proposed system.