A cord winding device for a window covering includes a casing, a reel rotatably mounted on the casing, a torsion spring biased between the casing and the reel, a limit ring secured on the casing and abutting the reel to limit the reel between the casing and the limit ring, and a pull cord attached a bottom of the casing. Thus, the lift cord of the window covering is wound around and stored in the reel of the cord winding device so that the lift cord is rolled up to a higher position to prevent the children from being tangled with the lift cord so as to protect the safety of the children.