The invention provides a processing method for denim yarns. The processing method is characterized by comprising the following steps: treating raw cotton through a cotton catcher, a metal removal device, a comb needle roller opener, a cotton adapter, a two-cotton-box cotton feeder, a rolling machine, a cotton combing machine and a rotating cup spinning machine in sequence to obtain the denim yarns, wherein the cotton combing machine comprises a buncher; the buncher comprises a web impurity absorption cavity; the rear end of the web impurity absorption cavity is connected with a web inlet; the front end of the web impurity absorption cavity is connected with a receiving opening; an impurity absorption opening is formed in the middle part of the web impurity absorption cavity; a filtering net is arranged at the impurity absorption opening; the impurity absorption opening is connected with an air suction device; the receiving opening is formed in a negative pressure cavity; a horn-shaped opening is formed in the front end of the negative pressure cavity; the buncher is arranged between an upper milling roller and a lower milling roller as well as between an upper pressing roller and a lower pressing roller; a web inlet is formed in one side close to the upper milling roller and the lower milling roller; the horn-shaped opening is formed in one side close to the upper pressing roller and the lower pressing roller. The method disclosed by the invention is easy for modifying, low in modification cost, obvious in effect and suitable for industrial production.