A flat membrane support plate includes a connection portion, a honeycomb structural portion and a support portion. The connection portion is configured to connect a diaphragm to the flat membrane support plate in a sealing manner, is arranged at a periphery of the flat membrane support plate, and is provided with at least one water outlet. The honeycomb structural portion is arranged on the flat membrane support plate in an area enclosed by the connection portion, is provided with a first flow channel for communicating with an interior of the entire honeycomb structural portion and communicating the honeycomb structural portion with the water outlet. The support portion is configured for reinforcing the honeycomb structural portion and is arranged between the honeycomb structural portion and the connection portion.