The invention relates to water conditioning constructional work in a water purification plant and particularly relates to an air-flotation and membrane-filtration purification system which is divided into a water buffering region, a water distribution region, an air-flotation contact region and an air-flotation separation and membrane-filtration water outlet region, wherein a micro-bubble generating device and a gas-solublized water conveying pipeline are connected and distributed in the air-flotation contact region; the air-flotation separation and membrane-filtration water outlet region is provided with a conical mud bucket, a supporting beam and an immersed membrane module from bottom to top; a slag scraping machine is capable of walking on a guide rail distributed on the side wall of a pool; a slagging groove is formed on a side wall opposite to the water inlet of the pool. The air-flotation and membrane-filtration purification system has the advantages of organically combining an air-flotation process with an ultrafiltration membrane filtration process, being capable of treating low-turbidity or high-algae water, and meanwhile being small in occupied space, simple in structure, low in civil engineering construction cost, high in load resistance capacity, low in auxiliary water ratio, low in operating cost and simple in automatic control.