Objective To explore the reasons for the failure of cathodic protection (CP) of long-distance pipeline. Methods The pipe on/off-potential test, potential distribution measurement and insulated flange test were adopted to investigate the protection status of the gathering pipelines, the direct current effects on the cathodic protection and the operation status of the insulated flange. Results The on-potential values of pipelines of 1# gas gathering station-1# valve chamber met the national standard within the range from -850 mV to -1200 mV. However, the off-potential values were in the range of -773~788 mV, which conflicted with the principle that requested the value to be lower than -850 mV. Although the on/off-potential of outside station increased with the increasing cathodic current in the inside station, the off-potential was still in the protection range of -850~1200 mV with the values of -880 mV and -1198 mV, respectively. As a result, overprotection was not observed and the national standard was strictly kept. The interference of CP was supposed to exist both inside and outside of the gathering station. We could adjust and balance the output inside and outside the station to make the protective potential of the pipeline reach the standard value within the range from -850 mV to -1200 mV, such as decreasing the output current of anode close to pipe outside the station, increasing the output of anode far from pipe of the station. Insulation flange performed well in the terminal station, but the grounding zinc battery was depleted. Conclusion The CP off-potential in the terminal station didn’t meet the requirements. The interference of cathodic protection systems between the inside and outside of the terminal station was the main reason for the failure of the cathodic protection of the long-distance pipeline.