In this paper, an algorithm for adaptively selecting suitable image block and filter sizes according to the real width of the fingerprint in an image is proposed. The problems that the direction information is incorrect due to the unsuitable image block size, the broken lines could not be connected or the connection of the endpoint with its neighboring fingerprint line are filtered mistakenly into bifurcation due to the unsuitable filter, in the existing directional filter methods, are solved. The validity of the algorithm has been shown in a lot of experiments.