A fuel injection pump having a separable plunger includes a separable plunger of which outer circumference is slide coupled to an inside of the a such that a lower portion thereof is supported by an upper portion of a main plunger to enable upward and downward slide movement in association with the main plunger, wherein an injection timing control spiral step of which upper surface is formed in a spiral shape, is protrudingly formed on the outer circumference thereof and a fuel passage is formed on one side of the outer circumference thereof including the injection timing control spiral step and an elastic member having a lower portion elastically supported by the separable plunger and an upper portion inserted within the barrel to be supported by an upper surface of the barrel.