The invention provides an on-line evaluation method and device for a dynamic performance of a thermal power plant excitation system in multiple scenes. A three-phase terminal voltage of a thermal power generating unit is obtained by a wide area monitoring system and a positive sequence voltage of the thermal power generating unit is calculated; a working system of an excitation system of the thermal power generating unit is analyzed and whether the excitation system of the thermal power generating unit is in an abnormal working state is determined; and a dynamic performance evaluation index of the abnormal working state of the excitation system of the thermal power generating unit is calculated and on-line evaluation is carried out on the dynamic performance of the excitation system of the thermal power generating unit. Besides, on-line evaluation is carried out on a damping characteristic of the excitation system of the thermal power generating unit during a low-frequency oscillation process and a motion performance of an excitation limiter of the excitation system of the thermal power generating unit and on-line identification and trouble shooting are carried out a generator set with an abnormal operating state or a difference with an expected response, and thus the poor dynamic performance of the generator set can be discriminated timely, so that secure and stable operation of the grid can be guaranteed effectively.