Based on the sideband spectrum produced by the interference of dispersion wave and solition wave in the passively mode-locked fiber laser. The wavelength offset from the central wavelength in the sideband spectrum is measured, the total cavity dispersion is obtained with the relation between the sideband offset and the total cavity dispersion. Ring cavities with different lengths of the same type fibers have different dispersions. The slope of the variation of dispersions corresponding to the fiber length is the dispersion coefficient to be measured. The platform of a passively mode-locked fiber laser is built, and the dispersion coefficient of G.652 fiber has been measured, whose experimental value is 16 ps/(nm·km) that is agrees well with the conventional value of 17 ps/(nm·km).