Project-based academic report writing (PBARW), one form of integrated writing, is widely used in Asia. We tested the hypothesis that L2 English learners have to achieve a threshold level of reading comprehension for PBARW strategies to have noticeable effects on writing performance. Quantitative and qualitative research methods were used in the study. Data were obtained from a questionnaire survey of 165 participants and learning journals from 16 participants. Participants' reading scores, strategies used, and PBARW scores based on their written products of around 2400 words were subjected to descriptive, correlational, and segmental regression analyses. A discernible reading comprehension threshold at the level of advanced B2, according to the Common European Framework of Reference for Languages (CEFR), was found to divide the effects of strategies on writing performance. Only strategies used by good readers predicted PBARW performance. Participants' learning journals also supported the conclusion that reading ability influences learners' competence of using reading/writing strategies. The findings have theoretical and pedagogical implications for PBARW teaching.