Abstract: An equivalent principle of noise masking and a noise index are suggested in this presentation, which are based on the characteristics of noise with wide frequency band and masking effects of auditory sensation. With the combination of time-frequency analysis and statistics theories, the disgusting component leading to annoyance can be distinguished from overall noise. The annoy index can be used to indicate the disgusting degree of individual frequency component. Then, a procedure of evaluating noise quality for the noise with wide frequency band is established. The experimental study shows that the presented model can be used to identify the annoy frequency components from overall noise promptly and accurately, and the results greatly agree with the subjective evaluation.