Online peer feedback plays a critical role in collaborative learning. This process improves learning and helps both giver and receiver develop and refine their metacognitive knowledge. A deeper examination of the feedback types, specificity, and affective nature is needed to understand its impact on collaborative knowledge improvement. A mixed-method study was conducted to examine how online peer feedback supported collaborative knowledge improvement in a computer-supported collaborative learning (CSCL) environment. How the different types of formative peer feedback, the specificity and affective nature of the peer comments supported pre-service teachers' in improving their TEL design were examined. The results showed that the peer feedback supported pre-service teachers in improving the quality of TEL design measured by the TPACK framework. It was found that feedback that raised concerns about the work and suggestive feedback facilitated the further improvement of the work. Elaborated feedback rather than verification feedback with short responses also affected the responses to the feedback. The implications on how online peer feedback support collaborative knowledge improvement are discussed. • Online peer feedback can support pre-service teachers in improving the quality of TEL design measured by the TPACK framework. • Elaborated feedback promotes post-feedback behaviour and supports pre-service teachers' improvement. • Feedback type questions and concerns and suggestions could feedforward, leading to collaborative knowledge improvement. • The affective state of the feedback comments does not influence how the recipient responds to the feedback.