Abstract: In surveying buried metal pipeline potential versus the earth, it is unavoidable to contain IR voltage drop an often neglected or misunderstood part, which does not reflect the electrochemical process of corrosion. Therefore, in the case of cathodic protection, IR voltage drop has to be deducted from the survey of potential so as to evaluate the effect of protection accurately and obtain the polarization potential. For this reason, the article relates to the way to measure IR voltage drop (including the case of cathodic protection); analyses the distribution law of IR voltage drop; deals with the application of IR voltage drop in determining the types and degrees of corrosion. In the end, it centers on explanation of the auxilliary polorization-method, the principle of which ts to reduce as far as possible the resistance from the reference electrode to the intervals of metal earthing bodies, a method to reduce IR voltage drop so much that it can be neglected. Also presented in the article are the structure scheme of auxilliary electrode, test connection circuit, and the effective measures to enhance the measuring accuracy of the auxiliary electrode method. The practice on site confirms that, of more than 10 ways to eliminate IR voltage drop, the auxiliary -electrode method is the simplest and easiest one in engineering.