A flat knitting machine structure with an adjustable gap between two knock-over bits includes two needle beds and two cam systems. Each needle bed comprises a plurality of needles and a plurality of knock-over bits. Each needle comprises a butt. Each of the knock-over bits comprises a control butt. The two needle beds are disposed at interval so that the knock-over bits face each other to define a gap. The distance of the gap is equal to a space between two knock-over bits facing each other. Each cam system comprises a needle cam to provide the plurality of butts being placed and guide each needle to make a knitting stroke towards the gap, and a knock-over bit cam provides the control butts being placed. The knock-over bit cam is controlled to define a displacement stroke for driving the plurality of knock-over bits to change the size of the gap.