Abstract: The influence of the modal dispersion of a signal fiber on velocity measurement in VISAR is theoretically analyzed and discussed in detail. A mathematical analysis model about the interfering contrast decreased by model dispersion is set up. The dispersion contrast factor varied with fringe constants and velocity increment is calculated. The results show: (1) the larger the fringe constants, the smaller the influence of the modal dispersion of a signal fiber; (2) the larger the measured velocity ,the larger the influence of the modal dispersion; (3) the influence of the modal dispersion of a multimode step-index fiber is larger than one of a multimode graded-index fiber. After this research, the efficient methods that decrease the influence of the modal dispersion of a signal fiber is put forward. This play an important role in improving interfering contrast and measuring precision in VISAR.