Qos of broadband network is important. ISPs have some methods for evaluate it. But they are not efficient enough. This paper present a new method in which delay and scale of windows of TCP messages are used for evaluating performance of broadband. Comparison between delay or scale of window s of samples is the core idea of the method. There are two kinds of comparison.One is absolute terms of above-mentioned indexes,another is score,that is relative term. We can grade samples according to their scores. It is helpful for locating probably fault situation for ISPs.