The invention relates to the field of product machining machinery and equipment, in particular to a box feeding method of a boxing machine. The box feeding method of the boxing machine comprises the following steps that A, a plurality of overlapped carrier boxes are arranged in a carrier box storage space; B, a second set of fork plates are located under the lowest carrier box to support the multiple overlapped carrier boxes; C, a negative pressure device carries out negative pressure sucking on the bottom surfaces of the carrier boxes; D, the negative pressure device moves downwards for a certain distance H, and then keeps static; E, a first set of fork plates are inserted into and support the penultimate carrier box; and F, the second set of fork plates release the corresponding carrier box, the negative pressure device moves downward to suck the lowest carrier box and continues to move downwards, the lowest carrier box is conveyed to the next station, and the step C is repeated. The box feeding method of the boxing machine is specific to the condition that the carrier boxes are overlapped tightly or horizontal sides of the carrier boxes are not obvious, and can accurately separate the tightly overlapped carrier boxes.