The invention provides a simple and convenient biogas fermentation device, which mainly comprises a bottom box, a fermentation bag, a fermentation bag base and a pipeline, wherein the bottom box is a detachable box body which is composed of a bottom surface and four side faces perpendicular to the bottom surface and is formed with an opening in the top; the fermentation bag is of a multi-layer structure which is arranged inside the bottom box, an earth bag, a heaven bag and a frame are sequentially arranged on the various layers from outside to inside; the earth bag is formed with an opening upwards and is provided with the fermentation bag base at the bottom; the pipeline comprises a gas pipe and a water pipe; fermentation materials are accommodated inside the frame; and the heaven bag and the earth bag are made materials which are not easy to corrode and are easy to deform with change in pressure. The device disclosed by the invention can be disassembled at any time, can not occupy land for a long time and can greatly reduce invested cost, and the filling of the biogas fermentation materials, the detaching operation of biogas residue and the delivery of biogas can be simpler and easier. Furthermore, the invention also provides a biogas generating method which makes uses of the device.