The invention discloses a low-pressure film rainwater quick purification device which comprises a mixing basin, a sedimentation basin and a film group filtration basin, wherein rainwater and a chemical agent fully react in the mixing basin; the sedimentation basin sediments the rainwater reacted with the chemical agent; and the low-pressure film group filtration basin further filters the rainwater, so that the drained water can be directly discharged into a downstream water body or recycled. The size of the device can be designed according to the rainwater amount and is not limited by the rainwater amount; and moreover, a purification effect is good due to medicating, sedimentation and low-pressure film filtration. The idea of the device is that the rainwater can be quickly purified by medicating, sedimentation and low-pressure film filtration, so that the water amount entering a sewage factory is correspondingly reduced when the rainwater amount in a rainy season is large, and the treatment sewage load is reduced; and moreover, the pressure on a pipe network is reduced. The device has the characteristics of low cost, high purification speed, simple structure and the like.