The invention provides a yarn humidifying machine and a humidifying method thereof. The yarn humidifying machine comprises a conveying system, a humidifying system and a steam system, yarns to be humidified are arranged on the conveying system, and are conveyed into the humidifying system by the conveying system, and the steam system is communicated with the humidifying system. The yarns are humidified by the aid of the yarn humidifying machine by the yarn humidifying method, and the yarn humidifying method includes steps: conveying the yarns to be humidified into the humidifying system; regulating the temperature and the pressure in the humidifying system; and feeding steam into the humidifying system and humidifying the yarns via the humidifying system. The yarns are humidified via the humidifying system, accordingly, the yarns are consistently humidified before being dyed, the weight of original yarns can be calculated conveniently according to the moisture content of the yarns, dye is matched, and matching of the dye is optimal.