To provide the control method of air conditioning system for shortening the time required before the opening degree of a valve is adjusted to a proper opening degree when switching air volume as far as possible, and an air conditioning system.

When the air volume of a fan 4 is switched into strong wind and, thereupon, the past proper opening degree under condition of middle wind or weak wind is larger than the present proper opening degree under condition of strong wind, the past proper opening degree under condition of middle wind or weak wind is updated to the present proper opening degree. Further, when the air volume of the fan 4 is switched into weak wind and, thereupon, the past proper opening degree under condition of strong wind or middle wind is smaller than the present proper opening degree under condition of weak wind, the past proper opening degree under condition of strong wind or middle wind is updated to the present proper opening degree.