Background: Enhancing the effectiveness of online learning has become a key challenge with regard to the ability of blended learning to reach its full potential. However, mechanisms by which students' self-regulated learning (SRL) skills influence their online learning engagement in blended learning and subsequent learning achievement have yet to be explored fully. Objectives: This study aims to explore the mechanisms underlying the effects of students' SRL skills on online learning behaviour and learning achievement in blended learning. Methods: Based on students' self-reported data and learning behavioural data, this study employed a combination of structural equation modelling and learning analytics to conduct an in-depth investigation into the influence of SRL skills on online learning behaviour and blended learning achievement. Results: The results indicated that the duration spent on challenging mandatory tasks mediates the relationship between psychological engagement and learning achievement; Two SRL skills, task strategies and self-evaluation, played positive moderating roles in the relationship between psychological engagement and online behavioural engagement. Further analysis of behavioural sequences revealed that these two skills influenced students' behavioural patterns during video learning and their effectiveness in error attribution during exercises. Additionally, self-evaluation impacted the approach to quizzes and error correction during exercises. Takeaway: The findings of this research enrich and advance the understanding of characteristics and processes of SRL in blended learning contexts and have important implications for both instructional design and learning support in the context of blended K-12 learning.