The invention provides a multi-air-foot over-location horizontal coplane adjusting method of an air-suspending system, belongs to the technical field of multi-air-foot coplane adjusting and aims at solving the problems that due to over-location of an air-suspending system supported by multiple air feet, levelness adjustment of equipment and coplane adjustment of the mutlipel air feet are coupled mutually, and the adjustment difficulty is large. According to the method, the manner that the three air feet are used for conducting supporting to adjusting the levelness and adjusting the coplane through multi-air-foot suspension is adopted, so that levelness adjustment and coplane adjustment are decoupled, and fast levelness and coplane adjustment is realized. The method comprises the steps that a preliminary criterion of multi-air-foot coplane and the levelness of an equipment base is established first; the air-foot coplane is adjusted under an air-foot non-bearing state to enable all the air feet to be located in a suspended state; auxiliary supports are mounted to enable the gap between each air foot and an air floatation platform to be a preset height value; and finally, the three auxiliary supports are disengaged from the equipment base, air is supplied to all the air feet to enable the air feet to float up, and horizontal coplane adjustment is completed. The method is used for horizontal coplane adjustment of the multiple air feet.