A cord-winding device for a blind assembly having no pull cord includes a base seat defining a slide channel and having a bottom side configured to removably mount the blind assembly thereto, and an adjustment unit including a slide seat mounted slidably in the slide channel, a plurality of rollers mounted on the base seat and the slide seat, respectively, and a constant force spring having first and second ends respectively fixed to the base seat and the slide seat. A lift cord winds around the rollers and extends through the base seat. The lift cord is configured to extend through the blind assembly after extending through the base seat so as to be positioned to the base seat and the blind assembly.