The invention discloses a tightness-adjustable nursing brassiere, which comprises cups, under-bands and an adjusting device, wherein the ends, which are adjacent, of the two cups are connected in a seamed mode, and the under-bands are connected to the other ends of the two cups; back buttons are arranged at the ends, away from the cups of the under-bands; linings are arranged in the cups; shoulderstraps are connected between the tops of the linings and the under-bands; first snap fasteners are arranged at the sides of the shoulder straps; second snap fasteners are arranged at the tops of thecups, and the second snap fasteners and the first snap fasteners are cooperated in use; barrel bags are arranged at the bottoms of the under-bands along a length direction; the initial positions of the barrel bags are located at the sides, away from the cups, of the under-bands, while the ending positions of the barrel bags are arranged at the bottoms of the cups; and the ends of elastic ropes areconnected to the bottoms of the interiors of the barrel bags. According to the nursing brassiere provided by the invention, a purpose of adjusting the tightness of the brassiere can be achieved undera circumstance of not releasing the back buttons, so that the brassiere is quite convenient; and meanwhile, by releasing the first and second snap fasteners, breasts can be exposed, so that a baby, who needs to be fed, can suck the breasts.