The invention discloses an on-line testing device and an on-line testing method for a digital signal channel of a nuclear power plant. The on-line testing device for the digital signal channel of the nuclear power plant mainly comprises a test signal receiving module, a signal receiving module, a first signal processing module, a second signal processing module and a test result display module, wherein the first signal processing module is used for performing first logic processing on a digital signal and a test signal so as not to change the digital signal and transmit the processed signal in a digital signal channel to be tested; and the second signal processing module is used for performing second logic processing on the signal, the test signal and the digital signal which are processed by the first signal processing module to acquire a test result. The invention also provides the on-line testing method for the digital signal channel of the nuclear power plant. Through the device and the method, whether signal channel transmission is normal can be directly judged according to the result displayed by the test result display module under the condition of not influencing the digital signal; the method is simple and practicable; and signal transmission is safe and reliable.