Current spectral analysis for track irregularity is based on sampling data at a uniform interval, but the train is of fluctuation velocity, therefore the accuracy of routine spectral analysis is in doubt. A nonuniform sampling sequence was studied to set up the power spectral density(PSD) formula of nonuniform sampling signal. The new formula was applied to analysis the nonuniform sampling cases when the sampling interval is of the Gaussian distribution probability density function. The result shows that the PSD formula of nonuniform sampling signal includes the classic uniform sampling signal’s PSD formula, which verifies the accuracy of the new formula. Finally a method was put forward to measure the probability density function of the sampling interval for the track irregularity spectral analysis.