A dog excrement automatic receiving assembly, an excrement device, a fixing piece and an excrement bag are provided. The dog excrement automatic receiving assembly at least comprises the excrement device, the excrement bag and a fixing piece. The excrement device suitable for wearing on a dog comprises a soft pad body, a hard combination part, a curtain and a plurality of connecting belts. The combination part, which is approximately located in the middle of the pad body, combined together with the pad body and fixedly combined with a first magnetic suction piece, has a through hole penetrated through both ends thereof and is covered by the curtain. The curtain comprises a first end which is fixedly combined with one side edge of the combination part and a second end which is a free end to be able to arbitrarily swing and fixedly combine with a second magnetic suction piece, in which the first magnetic suction piece fixedly combined with the combination part is corresponding to the second magnetic suction piece. The excrement bag comprises a sealed bag body which comprises an upper end provided with an inlet and first and second detachable extension parts respectively formed at two sides of the inlet. The fixing piece is an excrement fixing piece which is utilized to fixedly clamp the two extension parts of the excrement bag and is configured with a through hole located at the middle thereof and corresponding to the inlet of excrement bag. The bag body can be folded as a flat laminated shape to be assembled to the combination part together with the excrement bag fixing piece. With the second magnetic suction piece located on the curtain and the first magnetic suction piece located on the combination part to be temporarily combined to each other by mutual magnetic suction, the folded laminate-shaped excrement bag can be limited between the combination part and the curtain to be unable to open by the curtain. While the dog defecates, a push force exerted by the dog ex
