The bias magnetic field is related to the coupling efficiency of magnetostrictive effect and its inverse effect sensors. In order to meet the need for longitudinal mode magnetostrictive guided wave inspection, a method was proposed to evaluate the bias magnetic field produced by permanent magnets. Firstly, from the point of energy conversion in magnetostrictive guided wave defection process, the connection between the bias magnetic field and amplitude of guided wave signal is obtained. Secondly, a method was proposed to estimate bias magnetic field considering the uneven distribution of the magnetic field and the principle of magnetostrictive effect and its inverse effect. Thirdly, a finite element model is built and experiments are carried out to confirm the feasibility of the characterization of bias magnetic field. At last, a verification experiment was conducted to confirm the effectiveness of method. This method takes advantages of the values of magnetic field intensity along the circumferential direction on the pipe outer surface to characterize the bias magnetic field, using the relationship lines between signal amplitude and bias magnetic field. As a consequence, this method can be used to estimate the bias magnetic field for improving the efficiency of magnetostrictive longitudinal guided wave testing and provide a basis for the future study of intelligent instrument.