A yarn feeder of single-strand rubber yarn includes a supporting rack, at least one driving wheel, a transmission shaft, two bearing boards, a mandrel, a gear assembly, two supporting rods and a belt pulley module. The driving wheel is connected to an upper end of the transmission shaft. The transmission shaft is pivotally connected to the supporting rack. The two bearing boards are arranged separately. The mandrel is arranged between the two bearing boards, and two ends of the mandrel are pivotally connected to the two bearing boards. The gear assembly is arranged between a lower end of the transmission shaft and the mandrel. The two supporting rods are disposed between the two bearing boards, and two ends of each supporting rod are pivotally connected to the two bearing boards. The belt pulley module is coupled between the two supporting rods and the mandrel. Therefore, the two ends of the supporting rod have supporting point with stronger supporting strength. The supporting rods are not easily trembled, so that the feeding tension of yarn is steadier.