The invention provides a time-delay characteristic testing device and method of a digital substation secondary signal transmission system. The time-delay characteristic of the digital substation secondary signal transmission system, namely any node between output of a merging unit and an intelligent electric device, can be tested. According to the time-delay characteristic testing device and method, an Ethernet interface and an optical fiber interface are compatible, a time-delay test signal with periodically-changed amplitude and the stable frequency is generated through a three-phase controllable amplitude modulation power source, the cyclic wave position is calibrated through the amplitude, and interference of whole cyclic wave time delay in a time-delay test is eliminated; a self-generating synchronous clock signal is adopted, strict synchronization of inner sampling and signal circuit sampling is guaranteed, and the time-delay characteristic detecting accuracy is guaranteed; the corresponding relation between an inner sampling point and a signal circuit sampling point is determined through window comparison, the time difference of the corresponding sampling points is computed in a compared mode to obtain time-delay characteristic information, and transmission time difference shaking information of a system can be reflected.