In the context of COVID-19, a mixed-mode teaching is explored. The reforms for teaching content and mode are introduced through a case of indoor air environment, which is a core curriculum for postgraduate students of HVAC program. The innovations include teaching content reconstruction based on online-offline mixed teaching mode, interaction design based on an intelligent teaching platform, and online-offline mixed-mode teaching design based on flipped classrooms. The ideological and political elements are deeply researched based on network resources for COVID-19, and are integrated into in a CFD simulation teaching case to reshape the teaching contents. Flipped classrooms are adopted in the process management for curriculum paper to realize mixed-mode teaching and improve students' learning achievement. A good teaching effect has been achieved after a semester of teaching practice.