Thinking skills play a critical role in determining language performance. Recent advancement in cognitive diagnostic modelling (CDM) provides a powerful tool for obtaining fine-grained information regarding these thinking skills during reading. Studies are scant, however, exploring the relations between thinking skills and language performance, not to mention studies examining the variation of this association with language proficiency. The current study explored this variation through the lens of the Island Ridge Curve (IRC). Drawing on an English reading test data by 2,285 students, we identified five thinking skills using CDM. Next, we followed guidelines of IRC and put students into four language proficiency groups to examine the relations of each skill identified through reading tasks to language performance across groups. Results of multi-group path analysis showed the effect of each skill identified through reading test fluctuated in the pattern of the IRC. The potential of IRC for examining the moderation of language proficiency on language factors is discussed.