A driving structure of rectilinear motion of a middle friction wheel of a servo cone-pulley-type infinitely variable transmission comprises an input wheel, a middle roller and an output wheel, wherein two ends of the input wheel and the output wheel are connected with a left side plate and a right side plate, the input wheel and the output wheel are in line contact with the middle roller individually, the middle roller is fixed on a support shaft through a conical roller bearing, the conical roller bearing is externally connected with a middle support, a through hole of the middle support is internally connected with a screw nut, a screw end of the screw nut is connected with a servo motor, an upper end and a lower end of the middle support are connected with an upper guide rail sliding block and a lower guide rail sliding block respectively, the upper and lower guide rail sliding blocks are installed on upper and lower guide rails, the upper and lower guide rails are connected on upper and lower support shafts, two ends of the upper and lower support shafts are externally connected with shaft end sleeves, and the shaft end sleeves are mounted in corresponding holes of the left and right side plates and are fixed. By the upper and lower guide rail support means, the middle friction wheel can bear larger loads, and the stability and reliability of a speed regulating system is ensured. The screw nut is driven directly for speed regulation, so a speed regulating process is fast and accurate, and the structure is compact.