Teacher scaffolding is crucial to promote students' cooperative learning processes, but little is known about the cognitive attention underlying teachers' scaffolding behavior. This study aims to investigate the scaffolding behavior teachers exhibit while intervening in students' cooperative learning and to explore the relation between teachers' scaffolding behavior and visual perception when applying supportive strategies. Four secondary teachers in China were studied during mathematics lessons using eye-tracking glasses complemented by an additional classroom camera with interviews. The results show that the teachers applied scaffolding strategies very often, followed by the usage of diagnostic strategies. Furthermore, they were more concerned about the cognitive aspects of students' learning processes than the social aspects. The applied strategies were significantly related to fixation patterns during scaffolding. Detailed analysis suggests that (1) the teachers diagnosed students' understanding mainly by observing the material with an attention or scan pattern; (2) during scaffolding processes, when applying a cognitive performance strategy, teachers paid attention to both the students and their material; (3) while regulating cognitive processes, most teachers scanned students' material quickly; and (4) teachers generally focused more on materials to provide interaction-related help than on students' facial expressions. Additionally, the study points out that teachers' perception skills and behaviors during class activities may be influenced by the task being performed, teachers' teaching experience, and their beliefs.