The invention relates to a weaving machine by means of air flow, and particularly relates to an automatic weft drawing system of an air jet loom. The automatic weft drawing system comprises a weft collecting device comprises a gas channel, a mandrel rotating mechanism, a negative pressure opening, a mandrel and an air cylinder I; the mandrel rotating mechanism and the mandrel are arranged in the gas channel, and the bottom part of the gas channel is provided with the negative pressure opening; the mandrel rotating mechanism comprises a gear II, a gear I and a gear rotary shaft; the gear rotary shaft is fixedly connected with the gear I, and the gear II is meshed with the gear I; the gear rotary shaft and the mandrel are arranged in the gas channel, one end of the mandrel is faced to the gear rotary shaft and contacts with the gear rotary shaft; the other end of the mandrel is faced to a piston rod of the air cylinder and contacts with the piston rod; the outer part of the mandrel sleeves with a mandrel sleeve fixedly, and the mandrel sleeve is connected with the frame through a reset spring I. When a short weft appears, the system can automatically and exactly draw the weft, improve the working efficiency of the air jet loom and reduces the work load and labor cost of workers.