The invention provides a control method and device of a fresh air system and the fresh air system. The control method of the fresh air system comprises the following steps: judging whether the concentration of indoor carbon dioxide is less than a preset concentration threshold value; if the second humidity is equal to or greater than the sum of the first humidity and the preset difference humidity, controlling a cooling water spraying device to stop spraying, turning on a fan, and setting the gear when the fan is turned on as the set gear; if not, turning on the fan and determining the gear when the fan is started according to the concentration of carbon dioxide; according to the gear when the fan is turned on, acquiring the cooling load required for fresh air cooling of the fresh air system; and controlling the cooling water spraying device according to the cooling load and the temperature of the cooling water. The control method and device of the fresh air system and the fresh air system of the invention can avoid the phenomenon that a large amount of water mist adheres to the bottom of the ventilation duct, and can prevent mildew in the ventilation pipe. The temperature of freshair supply can be reduced accurately. At the same time, the method has the advantages of simple realization and low cost.