A blind body collecting-expanding control device for window blinds comprises a head rail, a bottom rail, a cord-winding control device, a transmission assembly, and a blind body wherein the head rail has an accommodating channel defining one end thereon to receive the cord-winding control device therein. The cord-winding control device links in movement to the transmission assembly composed of support seats, a winding shaft, lift cords, an operational cord, and a sliding element so that the blind body can be easily actuated thereby to collect upwards or expand downwards in operation. Via the aforementioned structure, the pushing or pulling force exerted by hands reciprocally interacts with the cord-winding control device so that a user can freely adjust the blind body into a desirable position, achieving the best adjustment effect as well as easy and speedy assembly thereof.