In recent years, different researchers have been paying increasing attention to self-determination theory (henceforth, SDT). This study explores the impact of extensive and intensive reading instruction on learners' motivation to read, self-assessment, critical thinking, and academic wellbeing through the lens of SDT. Conducted at a large university in China, the research involved 126 participants, equally distributed by gender and aged between 18 and 29, who were enrolled in a General English Course. The participants were divided into three groups: two experimental groups and a control group. The first experimental group received offline extensive reading instruction, while the second experimental group received offline intensive reading instruction. The control group, on the other hand, received traditional teacher-fronted instruction with minimal emphasis on reading. Data collection involved narrative frames and semi-structured interviews, with a thematic analysis revealing significant differences in outcomes between the groups. The first experimental group showed notable improvements in their motivation to read, self- assessment abilities, and critical thinking skills. The second experimental group demonstrated enhanced critical thinking skills and academic well-being. In contrast, the control group exhibited no significant improvements across the studied dimensions. These findings certify the trans- formative potential of alternative reading instructional approaches in fostering critical educational outcomes. By situating these results within the framework of SDT, the study highlights the importance of supporting learners' intrinsic motivation through tailored instructional strategies. These insights hold practical implications for educators, advocating for the adoption of extensive and intensive reading instruction to enhance learners' academic and cognitive development.