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: d1i, outside diameter: d10) (111) made of a first polymer (refractive index: n1) in a cylinder reactor (inside diameter: d0i, d10≦d0i) and inserting a 2nd plastic tube (inside diameter: d2i, outside diameter d20, d20≦d1i) (112) made of a second polymer (refractive index: n2, n2≧n1) 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′≧n2) into the 2nd plastic tube, injecting a precursor of a fourth polymer (refractive index: n″, n1≦n″≦n2) into the space between the 1st and 2nd plastic tubes.