A needle plate girder locating mechanism of a flat knitting machine comprises a needle bed body, a ball screw, a ball screw seat matched with the ball screw, a back needle plate long location block and a location groove arranged at the corresponding position of the needle bed body and matched with the back needle plate long location block. One end of the back needle plate long location block is fixedly arranged on the ball screw seat, the other end of the back needle plate long location block is slidingly arranged in the location groove, a cross section of the back needle plate long location block is an inversed T, a needle plate location pressing block is arranged on a T-shaped step face of the back needle plate long location block, and the needle plate location pressing block is fixed at the corresponding position of the needle bed body. The needle plate girder locating mechanism is simple in structure, adopts the T-shaped back needle plate long location block, enables the needle plate location pressing block to be installed on the back needle plate long location block, strengthens stability of reciprocating motion of the back needle plate long location block, improves working accuracy, is applicable to improvement of product quality of the flat knitting machine and reduces operation troubles.