A mounting platform assembly for a stand device includes a connecting platform having two parallel engaging blocks projecting from a bottom surface thereof, each having a positioning hole and forming a slope on an outward face thereof; and a fixing base having two engaging ribs projecting from a top surface thereof, each forming a sliding groove on the top surface of the fixing base corresponding to the slope of each engaging block, and the fixing base having a positioning means including an engaging button and a push rod. When a conical portion of the push rod abuts against a bottom end of the engaging button, the other end of the engaging button projects from the top surface of the fixing base to engage with the positioning hole of the engaging block, thereby achieving the positioning and fixing of the connecting platform.