The present invention relates to a method for preparing a GI (graded index) plastic optical fiber preform having a radial refractive index gradient, the refractive index increasing gradually from the outer periphery of the preform toward the center thereof, comprising inserting a 1st plastic tube (inside diameter: d.sub.1.sup.i, outside diameter: d.sub.1.sup.o) (111) made of a first polymer (refractive index: n.sub.1) in a cylinder reactor (inside diameter: d.sub.0.sup.i, d.sub.1.sup.o.ltoreq.d.sub.0.sup.i) and inserting a 2nd plastic tube (inside diameter: d.sub.2.sup.i, outside diameter d.sub.2.sup.o, d.sub.2.sup.o.ltoreq.d.sub.1.sup.i) (112) made of a second polymer (refractive index: n.sub.2, n.sub.2.gtoreq.n.sub.1) in the 1st plastic tube, the reactor, the 1st and 2nd plastic tubes are positioned coaxially, injecting a precursor of a third polymer (refractive index: n', n'.gtoreq.n.sub.2) into the 2nd plastic tube, injecting a precursor of a fourth polymer (refractive index: n'', n.sub.1.ltoreq.n''.ltoreq.n.sub.2) into the space between the 1st and 2nd plastic tubes.