According to Chaos Theory,the time series of NH₃—N pollution index at Minjiang Bridge in Le-shan City,Sichuan Province was reconstructed through the phase spaces.The reconstruction parameters were determined by the application of mutual information and Cao's method,and the characteristic indices including correlation dimension,Maximal Lyapunov index and Kolmogorov entropy were calculated accordingly.All the results indicate that a nonlinear chaotic characteristic can be clearly identified in the time series of NH₃—N pollution index of Minjiang Bridge,which might result from the non-linear chaotic dynamic system evolution.The research laid the basic foundation for further study on the complexity and evolutionary law of the time series of NH₃—N pollution index.It also offered the decision-making support for monitoring and protection of water quality in the middle and upper reaches of Minjiang River.