To succeed in integrated writing tasks, source use is commonly regarded as an essential but demanding skill for students at different educational levels. Whereas substantial efforts have been devoted to English settings, it is less clear how students use sources in non-English contexts. This study investigated source use (i.e., source idea use and linguistic transformation) in a Chinese integrated commentary writing task undertaken by 223 secondary students in Hong Kong. Textual analysis showed that for source idea use, secondary students tended to borrow ideas from source texts more frequently than they did when generating original ideas. As for linguistic transformation, students showed a significantly higher frequency of copying information from sources than paraphrasing. Furthermore, we found a significant impact of source use on writing performance, accounting for approximately 10% of the variance in task performance. Additionally, the predictive effect of the total number of original ideas on writing performance became significant after the total number of ideas from both source texts entered the model. This study provides a comprehensive understanding of secondary students’ source use in a Chinese integrated commentary writing task, with implications for integrated writing instruction and assessment. Copyright © 2025 The Author(s), under exclusive licence to Springer Nature B.V.