A lid of gas pressure regulator has a top segment, a surrounding wall, a pressing segment, and a gap. The surrounding wall protrudes downward and annularly from an annular edge of the top segment. The pressing segment protrudes transversely, outward and annularly from a bottom edge of the surrounding wall, and extends upward. The gap is formed between the surrounding wall and the pressing segment. Because the pressing segment is thinned and the gap is formed between the pressing segment and the surrounding wall, the overall volume and the weight can be reduced, and the pressing segment has sufficient height to prevent an opening edge of the seat from being bent at a large angle and fracture.