The invention discloses a nuclear power plant fuel assembly breakage on-line detection device and method. The device includes a pressure vessel communicated with an air suction port, a connection pipeline between the air suction port and the pressure vessel is equipped with a regulating valve and a gas power source, the device also comprises a data acquisition unit matching the pressure vessel, and a data processing terminal. The pressure vessel is communicated with an air outlet through an exhaust pipeline, and an isolation valve is installed on the exhaust pipeline. The device and method provided by the invention adopt a collection type measurement way, ensure maintenance of to-be-detected gas from a fuel assembly always in the pressure vessel before completion of the assembly measurement process, prolong the effective detection time of a detector on the gas, and greatly reduce the undetected probability of broken fuel assemblies. The device itself no longer needs an external gas source for gas supply, and compared the conventional detection way, the device has higher stability and higher detection accuracy and sensitivity.