The invention provides an on-line removal device for pressurized water reactor fuel assembly fission gas. The device consists of a fuel assembly, an upper buffer container, a flow regulating valve, acirculation pump, a fission gas adsorption apparatus, a helium gas supply apparatus and a lower buffer container. The upper part of the fuel assembly is connected to the upper buffer container, the upper buffer container is connected to the flow regulating valve, the flow regulating valve is connected to the circulation pump, the circulation pump is connected to the fission gas adsorption apparatus, the fission gas adsorption apparatus is connected to the lower buffer container, the helium gas supply apparatus is connected between the fission gas adsorption apparatus and the lower buffer container, and the lower buffer container is connected to the bottom of the fuel assembly. In the pressurized water reactor running process, the on-line removal device provided by the invention can achieveon-line removal of fission gas generated in the fuel assembly, improve the thermal conductivity in the enclosure gap, reduce the neutron poison in the reactor running process, and enhance the economical efficiency of the reactor.