The mixing system comprises: - a pipe (1) having a longitudinal axis (2), in which exhaust gases can flow in a flow direction (FD); - a nozzle designed to inject a fluid inside the pipe (1 ) from an injection inlet (5) arranged in the pipe wall, according to an injection direction (ID); - a first mixing device (6) positioned inside the pipe (1) upstream from the injection inlet (5), said first mixing device including a peripheral portion comprising blades (14, 15) capable of creating a peripheral swirl along the pipe wall, and a central portion designed to create substantially no turbulence; - a second mixing device (7) positioned inside the pipe (1) downstream from the injection inlet (5), said second mixing device (7) including a central portion (26) comprising blades (27, 28) capable of creating a swirl inside the pipe (1).