In this paper, a simulation model was built to study the CO 2 -based outdoor air rate control system in public buildings by PID control and fuzzy-PI control method. It is shown that classical PID algorithm can eliminates steady-state error, but the overtune is big and transitional time is long. The PID algorithm only suits for the case of CO 2 concentration changed small. When the CO 2 concentration has big change, fuzzy-PID algorithm can trace the CO 2 concentration, and control the outdoor air rates reasonably. The VAV system of public building employed fuzzy -PID algorithm can obtain a better control performance than that classical PID algorithm.Index Terms-public building ， CO 2 concentration ， outdoor air rate，fuzzy-PID control，simulation