A tunnel raceway type ball screw assembly includes a screw nut having an inner thread groove, a screw shaft inserted through the screw nut and including outer grooves, a mounting groove extending across the outer grooves and a first reflow groove in radial communication with the mounting groove, and a reflow component mounted in the mounting groove of the screw shaft and including a rolling groove connected with the outer grooves to form an outer thread that defines with the inner thread groove a spiral channel therebetween and a second reflow groove that defines with the first reflow groove a reflow channel therebetween in communication with the spiral channel. Thus, multiple balls can be cycled in the spiral channel and the reflow channel.