A bicycle hydraulic assembly includes a casing, a first piston, and a second piston. The casing has a hydraulic chamber, an inlet channel, and an outlet channel connected to the hydraulic chamber. The first piston is located in the hydraulic chamber. The second piston is sleeved on the first piston. The second piston has a first end portion and a second end portion opposite to each other. The first end portion is wider than the second end portion located farther away from the inlet channel than the first end portion. When the first piston is moved a distance smaller than a threshold distance, the first piston is moved relative to the second piston while the second piston is stationary relative to the casing. When the first piston is moved a distance greater than the threshold distance, the first piston forces the second piston to move relative to the casing.