The invention relates to a device and process permitting of the bottom drainage of a fluid-bed polymerization reactor (1) comprising a fluidization plate (2), a gas inlet (3) located between the said plate and the bottom of the reactor and a side pipe (4) located above the said plate and permitting of the evacuation of the fluidized particles, the said device consisting:of a vertical pipe (5) having a top opening at the level of the plate and a bottom end located outside and underneath the bottom of the reactor, which is occluded by a cock (6),and a gas intake tube (7) in the said vertical piping, leading out into the immediate vicinity of the said cock (6).