The invention discloses a membrane assembly for treating sewage. The membrane assembly for treating sewage comprises four independent membrane assemblies that are connected one another through connecting plates; a baffle plate is arranged on the lateral surface of each of the combined membrane assemblies, and each independent membrane assembly comprises a fiber membrane sheet, a fixed part and a pipeline joint, wherein two ends of the fiber membrane sheet are connected with the fixed part; the fixed part comprises a fixed part connecting end and a fixed part water outlet; a connecting housing is arranged out of the fixed part connecting end and the fixed part water outlet; the connecting housing out of the fixed part is connected with the pipeline joint; the fiber membrane sheet is formed into a rectangular shape, and the membrane in the fiber membrane sheet is made from a polyethylene material; pipeline joints of a plurality of combined membrane assemblies are connected with a pipeline; the fiber membrane sheet passes through the fixed part connecting end and the fixed part water outlet, and water can enter into the fiber membrane sheet through the fiber membrane sheet and then enter into the fixed part, and can flow into the pipeline through the pipeline joints and then flow out through the pipeline.