An auxiliary control device is adapted to an air conditioning system. The air conditioning system includes a main machine and several vents. The auxiliary control device includes a vent unit and a sub-control unit. The vent unit has valves corresponding disposed on the vents, and a driver for actuating the valve to adjust an opening size of the vent through the valve. The sub-control unit includes a sub-memory for storing a control time table, a sub-timer for generating the time information, and a sub-controller for controlling the driver to adjust the opening size according to the control time table and the time information. Accordingly, the auxiliary control device is easily adapted to an air conditioning system existing in a user place and controls opening size of the vents in accordance with the control time table previously set by the user.