A collapsible lockable support frame assembly includes a support frame pivoted to the support arm of a stand and having two parallel sliding rails, a slide holder carrying a positioning member at the back side and coupled to and movable along the sliding rails of the support frame for holding a display device and a locking mechanism having a casing affixed to the support frame and an upper stop member and a lower stop member pivotally swingingly connected to the casing. When the support frame is turned from a vertical position to a horizontal position, the upper stop member and the lower stop member are forced by the gravity thereof to turn out of the casing so that the upper stop member stops the positioning member from outward displacement to lock the slide holder, facilitating packing and delivery and the lower stop member limits the inward displacement distance of the slide holder to prohibit the display device from hitting the support arm of the stand.