The high proportion of renewables and high proportion of power electronics have become the significant devices in the power systems. The instability of power systems caused by wideband oscillation has become one of the important problems which need to be solved. Nowadays, the Fast Fourier Transform (FFT) is generally used to analyze the frequency spectrum characteristics of the oscillation waveforms. The shortage of FFT is analyzed in this paper. Then the FFT and Short Time Fourier Transform (STFT) are used to extract the time-frequency spectrum of oscillation wave-forms. Firstly, the simulation model of offshore wind farms connected to modular multilevel converter based high voltage dc (MMC-HVDC) transmission is built in PSCAD/EMTDC. When the control parameters are changed suddenly or the short-circuit fault appears in the system, the time-frequency spectrum characteristics of the wideband oscillation are studied. The asynchrony of the wideband oscillation is presented, which can be used to study the oscillation propagation of the HVDC system.