A method for knitting a three-dimensional fabric with variable thickness through a flat knitting machine includes the following steps: moving two cam groups and driving a plurality of knitting needles to knit a first piece of knitting by a starting cam system; moving the two cam groups and driving the plurality of knitting needles to knit a second piece of knitting by a middle cam system; and moving the two cam groups and driving the plurality of knitting needles to knit a supporting yarn by two tail cam systems respectively. The tail cam systems control each of a plurality of knock-over bit cams to move according to a gap size corresponding to a knitting length of the supporting yarn, so as to promptly change a thickness of the three-dimensional fabric along the length change of the supporting yarn.