This paper presents a kind of fractional delay filter algorithm on the basis of Lagrange interpolation in order to improve delay-time precision and to simplify the structure of LMS array signals. A sort of fractional delay filter is designed according to approximative algorithm in frequency-domain, the digital signal can be delayed with a fraction of the sample interval by it, and it is widely applied to the subjects which need accurate delay-time. Simulation results show that the fractional filter can fulfill the different delay-time desire for synthesizing array signal with high precision.