Abstract: The chaotic theory and method of phase reconstruction were used to analyze signal time series of electrode displacement during spot welding.And some indexes such as optimal delay time, correlation dimension, embedding dimension, and Lyapunov exponent which can reflect the chaotic property of time series at utmost were estimated quantitatively from signals of electrode displacement. As a result, the chaotic properties were discovered in these time series.And the comparison were done among the three methods:energy spectrum of wavelet, fractal dimension and Lyapunov component.The results show that the Lyapunov component can reflect the change of nugget size more accurately.