The invention relates to a three-section all-in-one rectangular anaeroic digestor comprising a distribution region, a reaction region and a three-phase separation region, wherein the cross section of the digestor is rectangular and respectively provided with the distribution region, the reaction region and the three-phase separation region from bottom to top, and the distribution region, the reaction region and the three-phase separation region are integrated; the distribution region comprises a bottom plate, the lower part of a side wall and a feeding pipe, wherein the upper surface of the bottom plate is connected with the side wall lower part; the reaction region is defined by the middle of the side wall, two sides inside the reaction region are oppositely provided with a first baffle plate and a second baffle plate from bottom to top, the reaction region is divided into three region chambers, and a gas collecting chamber is arranged at the upper part of each region chamber; and the three-phase separation region comprises the upper part of the side wall, an overflow veir, a three-phase separation baffle, a restricted baffle and a top cover, wherein the top cover is respectively provided with three gas collecting holes corresponding to the three gas collecting chambers, and the gas collecting holes are used for collecting the generated biogenic gas. The three-section all-in-one rectangular anaeroic digestor is compact in structure, convenient for manufacturing, easy to realize industrialization, uniform in distribution, not easy to block, high in treatment efficiency, good in liquid-solid mass transfer effect, high in organic load impact resistance and also capable of fractionally collecting the biogenic gas.