A collapsible support frame assembly includes a support frame having two parallel sliding rails, a slide holder coupled to and movable along the sliding rails of the support frame, a constant force spring mounted between the support frame and the slide holder, and a locking device having a casing affixed to the support frame and a stop member pivotally mounted in an accommodation chamber inside the casing. When the support frame is turned from a vertical position to a horizontal position, the stop member is forced by the gravity thereof to bias out of the accommodation chamber relative to the casing and to stop against the slide holder, thereby locking the slide holder, facilitating packing and delivery.