The invention provides an environment-friendly super-flexible bottom elastic layer composition for athletic tracks, made from an NCO component and an OH component in a weight ratio of 1:3 to 1:6, wherein the NCO component comprises, by mass, 15-30% of an isocyanate, 0-20% of an environment-friendly diluent,45-80% of a polyol, and 0.2-1.5% of a coupling agent; the OH component comprises, by mass, 5-15% of a polyol, 10-40% of an environment-friendly diluent, 1-5% of an environment-friendly chain extender, 40-70% of a filler, 0.2-1.5% of a pigment, 0.1-2.5% of a catalyst, 2-10% of a flame retardant, 0.1-1% of a dispersant, 0.1-1% of a leveling agent, and 0.1-1% of a defoaming agent. The bottom elastic layer composition prepared herein is environmentally friendly and has very high flexibility;the hardness of a plastic track is decreased evidently, with its elongation increased greatly; for regions having great day-night temperature differences and regions with expansion joints under the condition of zero track 'tearing', the athletic track has good elasticity, buffer for motion is improved effectively, and injury to the ankles, joints and tendons of athletes due to the reaction forceof the ground is alleviated; accidental injury in sports due to falls can be alleviated effectively.