This study reports on how the supplementation of online resources, informed by systemic functional linguistics (SFL), impacted English-as-a-foreign-language (EFL) student writers' development of critical thinking skills. Through qualitative analyses of student-teacher interactions, interviews with students, and students' written documents, the case study shows that through 1 semester of intensive exposure to SFL-based online resources in a college Chinese EFL writing classroom, EFL writers were able to develop critical thinking skills in regard to the construction of effective academic writing, although it was a process of encountering and overcoming challenges. Through teacher mediation and their own efforts, they could adjust to the online resources-based classroom, exemplified by their utilization of SFL-related categories offered through online resources to analyze and evaluate the interrelationship between language features and the content manifested in valued texts, and regulate the content of their own academic writing.