In L2 pedagogy, collaborative writing has been employed as a task for facilitating language learning, and language proficiency is often taken as a primary yardstick for pairing learners. Yet studies about the effect of proficiency pairing on language learning in pair writing have obtained mixed findings, and few studies have examined how proficiency pairing affects learners’ scaffolding strategy use. The present study, following a sociocognitive approach, investigated the effects of proficiency pairing on Chinese EFL learners’ language learning and scaffolding strategy use in pair writing. The study recruited three groups, respectively, including four high–high pairs, four high–low pairs, and four low–low pairs. An analysis of the 12 dyads’ dialogues elicited from pair writing revealed that low–low pairs produced more language-related episodes (LREs) and resorted to more scaffolding strategies than high–high and high-low pairs did while high–high and high-low pairs successfully resolved more LREs than low–low pairs did. The findings were discussed with reference to such mediating variables as task nature, interaction pattern, and task orientation. Pedagogical implications were drawn in the conclusion.