The invention relates to a gravity hydrophobic dilatation system for a waste heat boiler. The gravity hydrophobic dilatation system comprises a fixed drainage pit (8), the depth of the fixed drainage pit (8) is 2 meters to 3 meters below a zero meter line (11) of a boiler room, an internal water accumulation collecting pit (7) with the depth being 1.5 meters is formed in the fixed drainage pit (8), a fixed drainage dilatation container (1) is arranged in the fixed drainage pit (8) and receives water coming from a high-pressure system hydrophobic pipe (2), a middle-pressure system hydrophobic pipe (3) and a low-pressure system hydrophobic pipe (4), a spraying pipe (12) is arranged in the fixed drainage dilatation container (1), a liquid level measurement point for controlling opening and closing of a power-driven closing valve (6) is arranged in the fixed drainage dilatation container (1), opening and closing of a water discharge pipe (5) of the fixed drainage dilatation container (1) are controlled by the power-driven closing valve (6), and the water discharge pipe (5) is of a pi-shaped structure. The gravity hydrophobic dilatation system is used for gravity hydrophobic dilatation of the waste heat boiler.