To provide a padding method of a synthetic sleeper capable of forming a padding part for easily and accurately boring a prepared hole of a screw spike in a part straddling the padding part and a glass fiber reinforced thermo setting resin foaming body of a synthetic sleeper body.

In this padding method of the synthetic sleeper, a liquid urethane resin composition is filled in an unrequired hole bored in the synthetic sleeper and hindering driving of a rail fixing screw spike, the hole is padded by reacting and hardening the urethane resin composition in the hole, and the screw spike can be driven in the padding part. Soft material particles softer than a urethane resin are dispersively mixed in the urethane resin composition so that apparent hardness of the padding part after hardening becomes the almost same as hardness of the sleeper itself.