The invention provides a method for judging positive and negative pressure states of a fresh air system, the fresh air system and a medium. The method comprises the steps that the opening degree gearposition of a fresh air outlet or a fresh air return opening of the current area is adjusted; engineering equivalent mounting area parameters of the current area are calculated according to an air quality index before opening degree gear position adjusting and an air quality index after the opening degree gear position adjusting; and the positive and negative pressure states of the current area are determined according to comparing results of the engineering equivalent mounting area parameters and engineering actual mounting area parameters of the current area. Through the method, the problemthat in the related technology, judgment of the fresh air system on the positive and negative pressure states of the area is inaccurate is solved, and the judgment accuracy of the fresh air system onthe positive and negative pressure states in the area is improved.