A correct understanding of the mechanism of low frequency oscillation in the power grid is of great significance to the prevention and control of the production and dissemination of low-frequency oscillation. Based on an interregional low-frequency oscillation event in the Central China power grid recorded by the wide area measurement system（WAMS）， the Prony method is used to analyze the characteristics of various stages of oscillation. Combining with the small interfering frequency distribution statistical analysis， the forced oscillation theory is used to explain the oscillation mechanism. The results show that forced oscillation theory is able to reasonably explain the phenomenon of this oscillation at all stages， and quickly locating and removing the disturbance source is an effective measure to suppress such low-frequency oscillations.