Abstract: Ultra-low frequency oscillation（ULFO） refers to overall frequency oscillation caused by insufficient damping during the primary frequency regulation process. In general, there is no relative oscillation. However, in the study of the actual regional interconnected power grid, it has been found that, as the system frequency oscillates with ultra-low frequency, the same-frequency oscillation phenomenon also exists in the AC tie-line between two interconnected areas. Taking a two-area interconnected system as the research object, this paper analyzes the phenomenon and characteristics of tie-line power oscillation in ULFO based on the unified frequency model established by equivalent areas. Then the comprehensive frequency regulation effect coefficient of power grid is defined based on the damping torque theory. The mechanism of this phenomenon is proposed from the perspective of steadystate frequency response of linear system, and the influencing factors are also discussed. The research shows that, if there are differences in the comprehensive frequency regulation effect coefficients between two areas, they will also cause the samefrequency power oscillation of the inter-area tie-line as the steady-state ULFO, even after the ULFO reaches a steady-state oscillation. Finally, an actual power grid case is given for verification, and the possible harm of this phenomenon is discussed.