This paper discusses the reflection of chromatic dispersion in 40Gb/s system. And find a real time adaptive dispersion compensation technique based on BER feedback and fuzzy logic control. At the same time, the paper discusses the relation between BER and chromatic dispersion. In the paper, we do the simulation of 240 km (3 segments) 40Gb/s optical system. In the simulation, we can find the best chromatic dispersion tolerance of the 40Gb/s system. To compensate the CD, the fuzzy logic control is used. We give the fuzzy logic control arithmetic in the paper, put up the rule in the fuzzy logic control. We use the Bragg Gratings to compensate the CD. At last, we summarize the paper.