The invention discloses an active node for changing an octahedral truss unit into a geometric truss. The active node for changing the octahedral truss unit into the geometric truss is a cylindrical hemisphere combined compact active node, is of an upper and lower middle face symmetrical and angular bisector symmetrical structure and comprises a node central spindle sleeve, a central spindle screw, a spherical bearing and retainer, a screw type acting rod connection lantern ring, a first lug plate type acting rod connection lantern ring, a second lug plate type acting rod connection lantern ring, a first passive rod spherical bearing seat, a second passive rod spherical bearing seat, a needle bearing inner ring, a needle bearing, a needle roller thrust bearing pad, a needle roller thrust bearing and retainer, a deep groove ball bearing, a first semi-spherical nut and a second semi-spherical nut and can be connected with two extensible lengthened active rods and four fixed-length passive rods. Four fixed-length passive rods are subjected to motion decoupling, and the node is small, flexible in motion, large in strength and high in accuracy.