Abstract: In order to overcome the deficiency of single uniform fiber grating Sagnac loop, which has poor channel isolation, the comb filter with a cascade of Sagnac loop was proposed. Also, the frequency spectrum characteristic of the single uniform fiber grating Sagnac loop, two and three order Sagnac loop were analyzed with Jones matrix theory. For the case of three order Sagnac loop, a comb filter which has 0.8 mm spacing and 0.1 nm FWHM was got. Furtermore, in the same channel spacing, this comb filter's FWHM was only 1/4 that of the single uniform fiber grating Sagnac loop's. The results show that the system channel isolation degree is improved effectively, and a comb filter which has uniform passband interval, high channel isolation, narrow FWHM characteristics was proposed, this kinds of filter can meet the WDM requirement better.