A full-angle coincidence brain PET detector and apparatus, including a plurality of PET detection modules, each of which includes PET detection crystals, a photoelectric sensor array and light guides, all the detection crystals being arranged towards a cavity. The detection modules form a chamber having an opening, each dimension of the cavity is no greater than 35-50 cm, the opening is located at a lower side of the chamber. A cross-sectional area at the opening is greater than a maximum cross-sectional area of a head in a horizontal direction. Except for the opening, all the detection modules are non-detachably connected together. All cross-sectional areas of gaps between the detection modules are less than ½ to ⅓ of the area of the smallest one of the detection crystals. At least 75% to 80% of a coincidence event occurring at a center of the chamber is detected by the detection modules.