The invention proposes a grade-A voltage transient monitoring device, a method and a system. The grade-A voltage transient monitoring device comprises a sampling module, a harmonic wave analyzing module, a three-phase fundamental wave zero-crossing positioning module, a fundamental wave zero-crossing synchronous semi-wave effective value calculating module, a transient state start-and-end determining and residual voltage enduring time calculating module. According to the invention, the device, the method and the system of the invention directly use the real part and the virtual part of the fundamental wave components provided by the harmonic wave analyzing module for the synchronization of the fundamental wave zero-crossing and measure the enduring time based on the fundamental wave zero-crossing as well as the residual voltage. Compared with the use of a digital filter and the bisection method for the synchronization of the fundamental wave zero-crossing and the measurement of the enduring time based on the fundamental wave zero-crossing as well as the residual voltage, the procedures for the invention are simplified considerably, reducing the loads to the system and at the same time, realizing the accurate measurement of the residual voltage and the enduring time based on the fundamental wave zero-crossing.