The output signal of B-dot probe in the cavity increases at low frequency. The first-order correction method of “a characteristic flux-penetration time” is analyzed. According to the analysis, the method of compensation with integrator is proposed. Frequency response experiments were carried out for B-dot installed in the electrodes of coaxial and radial lines. The results show that when the RC time constant of the integrator is equal to the characteristic flux-penetration time, the measurements of B-dot coincide with the standard current signal. Therefore, for the B-dot probe in the cavity, this is the principle of selecting the RC constant of the integrator. The compensated test results are obtained directly by hardware, thus it is more convenient to operate than the software correction method.