A needle bed structure for flat knitting machines that is located at two sides of a needle bed gap includes a plurality of parallel needle plates, a plurality of knitting needles and sinkers located between the needle plates, a sinker control portion movable in parallel with the arranged direction of the sinkers, and at least one control cam driven by a switching portion to push the sinkers to rotate towards the needle bed gap during the switching movement. The switching portion performs a switching movement according to the moving direction. The needle plates hole at least one needle pressing board to restrict vertical movement of the knitting needles. The switching portion has a magnetic attracting portion corresponding to the needle pressing board to form magnetic attraction therewith. A spacer is provided between the needle plates to prevent the horizontal movement of the sinkers.