For nonstationary time series with outliers and strong noises, in order to increase the accuracy and robustness of wavelet threshold denoising in the low frequency signal decomposition, the arc-tangent function transformation is employed to transform the original nonstationary time series to a new more stationary series. The SNR(Signal-to-Noise Ratio)is improved and the negative effect of outliers is insulated. The suppressing capability of arc-tangent function transformation to outliers is explained analyt...