The reaction and extrusion polymerizing process of linear styrene/conjugated diolefin block copolymer with nanometer level dispersed phase of special structure and performance includes the following steps: feeding styrene monomer and conjugated diolefin monomer in certain ratio to the first section of screw extruder, feeding polymerization initiator to the second section, and adding antioxidant and other modifiers to the screw extruder from one side and extruding out. The styrene block copolymer has outstanding percentage elongation at rupture and excellent mechanical performance. The present invention combines polymerization reaction and extrusion to shorten the production period to several minutes, and the whole process has no solvent added and thus no need of complicated solvent separation and purification. Therefore, the present invention has high production efficiency, low power consumption, low cost and less pollution.