Forums are essential components facilitating interactions in online courses. However, in large-scale courses, many posts generated, which results in learners' difficulties. First, the posts are poorly organized and some deviate from the topic, making it difficult for learners' knowledge acquisition. Second, learners cannot receive timely feedback and guidance, making the learning progress unclear for them. Well-designed scaffoldings should be built based on challenges of forums to improve learners' learning outcomes, knowledge construction, and completion rate. While targeting the problems in online forums, this article proposed principles for the design of online scaffolding after analyzing the requirements of online learning scaffolding or scripts. Subsequently, in this article, we designed an automatic feedback scaffolding based on the principles and a knowledge construction model. The scaffolding provided learners with timely feedback and related learning guidance. Tags were used to assist learners in acquiring relevant information more easily. The scaffolding was then integrated into the Learning Cell Knowledge Community and used in an online course for 955 learners. The results showed that automatic feedback scaffolding positively affected learners' learning and promoted positive knowledge transformation. Furthermore, we found that the scaffolding could help learners induce more constructive behaviors defined in the Interactive, Constructive, Active, and Passive deep learning framework that demonstrated the reason for learners' knowledge transformation. At last, learners' course completion rate also increased with the help of the scaffolding, which provided evidence that well-designed scaffolding can result in positive educational outcomes. In addition, the principles proposed could also contribute to further scaffolding design and practices.