A paint cup for a spray gun, comprising: a container adapted for containing paint solvents and including a bottom wall, an annular side wall extending upward from a periphery of the bottom wall, and an opening in an upper end of the container; a lid body covering the opening of the container and including an inner surface facing the container and an outer surface opposite to the inner surface, with the lid body further including an annular coupling portion protruding from the inner surface of the lid body in an axial direction, with an inner rod extending downwardly from the inner surface of the coupling portion in the axial direction and located in the annular coupling portion, with the inner rod having a length in the axial direction greater than a length of the annular coupling portion in the axial direction, with an annular coupling groove formed in a bottom end of the coupling portion and defined between an annular inner wall and an outer periphery of the coupling portion in a radial direction perpendicular to the axial direction, with an accommodating groove formed between the inner rod and the annular inner wall of the coupling portion 34, with a vent extending from the outer surface through the inner surface of the lid body and into the annular coupling groove of the annular coupling portion; and a tubular ventilation member mounted around the inner rod and including an expansion portion at an upper end of the tubular ventilation member, with the expansion portion engaged in the annular coupling groove of the coupling portion, with a gap formed between the tubular ventilation member and the inner rod, wherein when the container is provided with the paint solvents and turned upside-down, the paint solvents in the gap between the ventilation member and the inner rod will flow into the accommodating groove of the coupling portion, and ambient air circulates in the container via the vent, the annular coupling groove, and the gap. a container adapted for containing paint solvents and including a bottom wall, an annular side wall extending upward from a periphery of the bottom wall, and an opening in an upper end of the container; a lid body covering the opening of the container and including an inner surface facing the container and an outer surface opposite to the inner surface, with the lid body further including an annular coupling portion protruding from the inner surface of the lid body in an axial direction, with an inner rod extending downwardly from the inner surface of the coupling portion in the axial direction and located in the annular coupling portion, with the inner rod having a length in the axial direction greater than a length of the annular coupling portion in the axial direction, with an annular coupling groove formed in a bottom end of the coupling portion and defined between an annular inner wall and an outer periphery of the coupling portion in a radial direction perpendicular to the axial direction, with an accommodating groove formed between the inner rod and the annular inner wall of the coupling portion 34, with a vent extending from the outer surface through the inner surface of the lid body and into the annular coupling groove of the annular coupling portion; and a tubular ventilation member mounted around the inner rod and including an expansion portion at an upper end of the tubular ventilation member, with the expansion portion engaged in the annular coupling groove of the coupling portion, with a gap formed between the tubular ventilation member and the inner rod, wherein when the container is provided with the paint solvents and turned upside-down, the paint solvents in the gap between the ventilation member and the inner rod will flow into the accommodating groove of the coupling portion, and ambient air circulates in the container via the vent, the annular coupling groove, and the gap.