The negative electrode of the battery is connected with the metal part of the body in the automobile, so the negative conductor on the automobile is usually called the tie line. The tie line plays an important role in the automobile circuit. Therefore, the quality of the ground rail is the key to the work of the automobile electric appliance. In repair work, it usually takes a lot of time to diagnose bad faults. This article introduces the function of automobile tie line and the analysis and diagnosis of common faults.