A hydraulic caliper brake assembly for a bicycle comprises a caliper body; a first chamber installed in the caliper body; a first piston installed in the first chamber; a second piston installed in the first chamber; an inner chamber formed between the first piston and the second piston for receiving brake oil; an outer chamber installed between the second piston and the caliper body for receiving brake oil; and an outer valve installed in the second piston for switching flowing of the brake oil between the inner chamber and outer chamber. When a small force is applied, the brake oil will flow into the inner chamber; by closing the outer valve, the first piston is pushed to have the effect of speed reduction; when the braking force is increased, by opening the outer valve, the second piston is pushed to have the effect of emergent braking.