Combining demand controlled ventilation (DCV) control and economizer control achieves acceptable and even better indoor air quality with minimum coil energy consumption. The control instability during the transition processes between different control modes are among the major difficulties faced when utilizing economizer control and when combining DCV control with economizer control in applications. A robust control strategy, using "freezing", gain scheduling, integral term reset and feedback transition control for different transition processes, is developed for addressing these problems. They are evaluated on an air handling unit (AHU) using various simulation tests. The test results demonstrate that the robust control strategy allows stable and robust AHU control.