An over-constrained few-joint five-degree-of-freedom hybrid robot, comprising: a fixed platform, vertical frames, a single-rotation-pair rotating frame, a double-rotation-pair rotating frame, a movable platform, a working platform, three branch chains (4, 5, 9) of the same structure and a two-degree-of-freedom posture adjustment head, wherein the working platform is disposed in front of the fixed platform; the vertical frames are symmetrically disposed on two sides of the fixed platform; the three branch chains (4, 5, 9) are disposed between the two vertical frames and the movable platform; the two-degree-of-freedom posture adjustment head is disposed at the lower end of the movable platform; a two-rotating and one-translating three-degree-of-freedom parallel mechanism is formed by the vertical frames, the movable platform and the three branch chains (4, 5, 9); wherein each of the three branch chains (4, 5, 9) of the same structure comprises a servo motor, a servo motor base, a coupling, a lock nut, a lead screw 19), a bearing, a bearing end cap, a lead screw nut, a connecting rod sleeve, a connecting rod, guide rails, sliders and slider caps, wherein the servo motor base with the servo motor disposed thereon is disposed at the top of one end of the connecting rod sleeve; in the connecting rod sleeve, an output end of the servo motor is connected to one end of the lead screw by the coupling; the lead screw nut mounted to one end of the connecting rod is disposed on the lead screw; an effective stroke part of the lead screw cooperates with the lead screw nut; the lead screw is supported by the bearing disposed at the bottom of the servo motor base; an inner ring of the bearing is locked by the lock nut; the bearing end cap is disposed on an inner side of the servo motor base and configured to secure an outer ring of the bearing; the other ends of the connecting rods of a first branch chain and a second branch chain are connected to the movable platform by two-degree-of-freedom Hooke joints respectively; the other end of the connecting rod of a third branch chain is connected to the movable platform by a rotation pair; parallel guide rails are disposed on two sides of the connecting rod; the sliders are disposed on the guide rails; a group of slider caps are symmetrically disposed on two sides of the lower part of the other end of the connecting rod sleeve; the slider cap is mounted to an end cap mounting surface of the connecting rod sleeve; the sliders are mounted to inner sides of the slider caps and are capable of sliding along the parallel guide rails on the two sides of the connecting rod, such that the connecting rod sleeve and the connecting rod constitute a prismatic pair; a dustproof end cap is disposed at the end part of the other end of the connecting rod sleeve; wherein a group of corresponding motion-pair supports that are fixedly connected to the vertical frames are disposed at the tops and in the middle of the two vertical frames; the double-rotation-pair rotating frame is disposed between the group of motion-pair supports disposed at the tops of the vertical frames; the single-rotation-pair rotating frame is disposed between the group of motion-pair supports disposed in the middle of the two vertical frames; the double-rotation-pair rotating frame and the single-rotation-pair rotating frame are connected to the motion-pair supports by rotation pairs; the one ends of the connecting rod sleeves of the first branch chain and the second branch chain respectively pass through through-holes in the double-rotation-pair rotating frame and are connected to the double-rotation-pair rotating frame by rotation pairs, and axes of the rotation pairs perpendicularly intersect an axis of the rotation pair between the double-rotation-pair rotating frame and the motion-pair support; and one end of the connecting rod sleeve of the third branch chair passes through a through-hole in the single-rotation-pair rotating frame to be connected to the single-rotation-pair rotating frame by a rotation pair, and the axis of the rotation pair perpendicularly intersects an axis of the rotation pair between the single-rotation-pair rotating frame and the motion-pair support.