The invention discloses a nuclear power plant closed circulation system backwashing device, which comprises a first short pipe and a second short pipe; the first short pipe is provided with a filter screen provided with a front end and a rear end, at least a first open hole is arranged at the periphery of the first short pipe, and a water inlet pipe with a flushing valve is installed at the firstopen hole; and the second short pipe is connected with the first short pipe, the second short pipe is provided with a second open hole, and a drain pipe with an isolation valve is arranged at the second open hole. Compared with the prior art, the nuclear power plant closed circulation system backwashing device can connect a backwash water source with pressure to a closed circulation system, flushing water backwashes the filter screen from the rear end of the filter screen to the front end of the filter screen, sediments in the filter screen flow out of the filter screen along with the flushingwater and are discharged through the drain pipe at the front end of the filter screen, and therefore online backwash without disassembling the filter screen can be achieved.