The invention belongs to the field of polymers, and particularly relates to an on-line polymer slurry sampling system. The system includes a polymerization kettle and a testing kettle. The bottom of the polymerization kettle is connected to a material discharging tank through a material discharging pipe. A solvent tank is connected to the material discharging pipe through a solvent washing pipeline. The solvent washing pipeline is provided with a metering pump and a first solvent washing valve. The material discharging pipe is connected to the testing kettle through a sampling pipe. The sampling pipe is provided with a sampling valve. The top of the testing kettle is provided with a pressure transmitter. An on-line particle analyzer probe is disposed in the testing tank, and is connected to a signal emitter and an analyzer computer in order. A device for implementing a method of detecting granularity of granules in polymer slurry on line is provided, automatic sampling in a polymerization process is achieved, analysis and detection speeds are high, and an operator can adjust the polymerization process conveniently in time. The system is performed under sealed conditions, and therefore no impurity is brought into the polymer slurry, and the slurry can be fed into the polymerization kettle and participate reactions after the slurry is recovered.