The invention discloses a traverse motion transmission device for a weft laying cart of a warp knitting machine. The device comprises a traverse motion support base in rolling or sliding fit with a warp knitting machine beam guide rail and a synchronous belt pulley arranged in the traverse motion support base; a driving device is fixedly connected with the upper part of the traverse motion support base, a transmission mechanism is arranged between the driving device and the synchronous belt pulley, and the driving device drives the synchronous belt pulley to rotate by the transmission mechanism; and, the upper part of the guide rail is provided with the synchronous transmission pulley along the length direction, a tooth form face of the synchronous transmission pulley is opposite to the upper surface of the guide rail, both ends of the synchronous transmission pulley penetrate two transverse sides of the traverse motion support base respectively and are fixedly connected with the guide rail, and the synchronous transmission pulley arranged in the traverse motion support base is meshed with the synchronous belt pulley. The device has the advantages of high precision, hard damage, low noise, long service life of the driving device, and good commercial prospect.