In recent years, the technique of through casing formation resistivity logging shows great practical value and practical significance in gas-water identification, residual oil evaluation, reservoir fluid saturation dynamic monitoring and other aspects. In this paper, based on the understanding of the resistivity measurement principle through casing, combining experiment with theory, using Hall Effect principle, the variation characteristics and influence factors of the resistivity measurement signal outside casing (simulating the resistivity measurement condition of through casing formation)under the presence of metal casing are studied. Through simulated experimental measurement, the response characteristics of external electric field of metal bushing under four kinds of ideal media conditions are compared and analyzed. It is found that there is an obvious correlation between measured voltage signal and current intensity under the condition of certain magnetic field intensity. The measured voltage changes with the change of the resistivity outside the casing, and the voltage increases gradually with the increase of the resistivity outside the casing. The measured voltage value is not only affected by the magnetic field strength, but also by the smoothness of casing wall, cement thickness and other factors. The experimental results provide a new idea and method for the development of domestic resistivity logging instrument through casing in the later period.