The utility model discloses a main device assembling type case of a fresh air system. The main device assembling type case comprises a rectangular bottom frame. A bottom plate is installed on the bottom frame. The four corners of the bottom frame are connected with four stand columns. A first baffle is connected between every two adjacent stand columns. A fixing frame for containing a draught fan is connected to the upper portions of the four stand columns, the upper ends of the four stand columns are connected with a top frame, the upper end of the top frame is provided with a second baffle. The main device assembling type case of the fresh system can be disassembled into a plurality of components, the transportation size and transportation cost can be reduced, the device can be protected easily, and the total weight can be reduced.