This paper aims to explore the design of an online learning assessment mechanism and its impact on teaching effectiveness under the blended learning mode. Through various assessment methods and personalized differentiated assessment content, the learning situation of students can be effectively understood, teaching effectiveness can be improved, and students' comprehensive development can be promoted. Research shows that an effective online learning assessment mechanism can help teachers better guide students, adjust teaching strategies in a timely manner, improve students' learning effectiveness and satisfaction. Therefore, designing an effective online learning assessment mechanism is of great significance for optimizing the blended learning mode.