An auxiliary mounting device and mounting method for a saddle-type window air conditioner. The auxiliary mounting device comprises a base and an airbag; the base is located on a window, an upper end of the airbag is connected to a connecting portion of the saddle-type window air conditioner, a lower end of the airbag is connected to the base), an air hole is provided on the airbag, and a plug is provided at the air hole. When mounting the saddle-type window air conditioner, the airbag is inflated first, and then the saddle-type window air conditioner, together with the base and the airbag, is placed on the window, the base sits on the window, then the airbag is deflated, and the connecting portion gradually descends, along with the deflation of the airbag, in the direction towards the base, thereby enabling the saddle-type window air conditioner to fall slowly. The auxiliary mounting device has a simple structure and is easy to operate.