The invention discloses a method for mounting a large gear applied to a flexible transmission device of a large-size sintering machine. The method comprises the steps of 1, cleaning the large gear and a shaft neck which is arranged on a main shaft and used for allowing the large gear to be assembled thereon; 2, checking whether a mounting hole of the large gear and the assembling dimension tolerance of the main shaft are within the preset fit tolerance scope or not, and if yes, implementing the following step; and 3, hoisting the large gear to the shaft neck of the main shaft. According to the method for mounting the large gear, the large gear is hoisted to the shaft neck of the main shaft through a hoisting mode; the problem that the large gear is impacted is avoided; and the end face of the large gear can be protected from damage conveniently.