An antilock brake system for a bicycle is mounted to a brake mounting bracket and generally comprising: brake shoe holder, sliding seat, brake shoe and elastic member. The brake shoe holder is formed with groove for slidably receiving the sliding seat, the brake shoe is disposed in the sliding seat, the brake shoe holder is provided with an antilock slope, and the sliding seat is correspondingly provided with a projecting portion for abutting against the antilock slope; the elastic member serves to retain the sliding seat at an end of the brake shoe holder in a rotating direction of a bicycle wheel before the brake is applied.