Based on the modular multilevel converter (MMC), flexible direct current (DC) transmission technology has been widely adopted in China′s power system. In recent years, from the Yu-Hu Project to the Zhangbei Project, high-frequency oscillation phenomena have occurred in multiple voltage-source converter based flexible DC transmission projects both domestically and internationally, jeopardizing the safe and stable operation of the system. To address this issue, this paper proposes a high-frequency oscillation detection method for flexible DC transmission systems based on wavelet packet decomposition (WPD) and unscented Kalman filter (UKF) tracking. First, the filter bank structure of fast wavelet packet decomposition (WPD) is employed to accurately partition the grid-connected current signal of the flexible DC system into frequency bands. Then, a threshold is set based on the wavelet packet coefficients of each node at every layer. If the threshold is exceeded, it is determined that a high-frequency oscillation may be present. The wavelet packet coefficients that exceed the threshold are reconstructed to obtain a time-domain signal containing only specific frequency components. Subsequently, the unscented Kalman filter (UKF) is applied to track this signal, yielding dynamic frequency tracking curves and dynamic amplitude tracking curves. Finally, an MMC-based flexible DC transmission system model was built on a simulation experimental platform to generate high-frequency oscillations. The results verify the sensitivity and accuracy of the proposed method in detecting high-frequency oscillations, meeting the detection requirements for subsequent oscillation suppression.