Abstract: The reasonable design of bias magnetic field is one of the key factors of longitudinal modes guided wave sensor based on magnetostrictive effect. The purpose of this paper is to study the distribution regularity of the bias magnetic field in large diameter pipes. Different magnetic circuits for an 88 mm diameter pipe was discussed based on numerical simulation and the distribution variance was used to measure the uniformity of the bias magnetic field. The result showed the distribution variance presented logarithm type attenuation with the increment of magnetic circuit number and finally a five-magnetic-circuit bias magnetic field was chosen for the magnetostrictive sensor.