A yarn conveying system includes a circular knitting machine, at least one yarn conveying tray and a control unit electrically connected to the circular knitting machine and yarn conveying tray. The circular knitting machine has a needle cylinder and at least one yarn feeder arranged annularly on the needle cylinder. The yarn conveying tray is wound by at least two separate yarns and includes a driving device to drive the yarn conveying tray rotating so that the yarns are supplied via a yarn conveying path to the yarn feeder. The control unit generates a yarn feeding signal to the circular knitting machine to control yarn feeding speed and a driving signal to the driving device to control rotational speed of the yarn conveying tray. Thus users can operate the control unit to control speed of the yarns supplied from the yarn conveying tray to the yarn feeder.