A weft yarn conveying thread control device comprises a speed changer and a control unit, wherein the speed changer comprises an input end shaft capable of receiving power output by a power source, a transmission unit connected with the input end shaft, a speed adjusting rod arranged in the transmission unit in an extending mode, and an output end shaft capable of connecting the power to a weft supply device. Output rotation speed of the speed changer is enabled to be controlled by swing angles of the speed adjusting rod. The control unit comprises a connection mechanism which drives the speed adjusting rod to swing relative to the transmission unit and a controller which controls the connection mechanism and can be controlled in a programmed mode. Matched with the control unit, speed change rate of the speed changer is changed according to required patterns, so that power of the power source is changed in speed by the speed changer and then is output through the output end shaft. Conveying speed of weft yarn is controlled in a driving mode, and occur of defects is reduced.