The invention discloses a multi-stage rainwater biological treatment device which comprises a presedimentation pool and a filter pool. A water inlet is formed in the presedimentation pool, a drainage opening is formed in the filter pool, and an overflow baffle structure is arranged between the presedimentation pool and the filter pool. A porous medium filter layer is arranged at the bottom of the filter pool, a biomedium layer is arranged at the top of the porous filter medium layer, a rainwater collecting pipe is buried in the porous filter medium layer, a plurality of rainwater inlets are formed in the pipe wall of the rainwater collecting pipe, one end of the rainwater collecting pipe is connected with a cleaning pipe, the other end of the rainwater collecting pipe is connected with a drainage pipe, and the drainage pipe is connected with the drainage opening. A cover plate is arranged at the top of a multi-stage treatment pool, and provided with a biomedium layer plant seed inlet. The multi-stage rainwater biological treatment device integrates gravity settling, screening, filtering and biological treatment functions to carry out multi-stage treatment on rainwater, after sediment of the rainwater is removed through the presedimentation pool, preliminary purification is carried out on the rainwater through the overflow baffle structure, further filtering and biological treatment are carried out on the rainwater through the biomedium layer, plants and the porous medium filter layer, and therefore pollutants are effectively removed.