In this paper, a simulation model was built to study the CO2-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 CO2 concentration changed small. When the CO2 concentration has big change, fuzzy-PID algorithm can trace the CO2 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.