Abstract: To eliminate the disturbances from the speed-instability of the measurement vehicle on the collected track irregularity spectrum, Haar wavelet transform was used to analyze the instantaneous frequencies of the track irregularity signals. The relationship between the instantaneous frequencies of the track irregularity signals and the Haar wavelet coefficients was established to calculate the instantaneous frequencies. The calculated instantaneous frequencies are used for interpolation to get the re-sampled signals, which are then Fourier-transformed to get disturbance-free frequency spectrum.