In this paper, one pipeline in the southwestern China was taken as the research object to study the influence of a UHVDC transmission system in the operation mode of monopole ground return on the buried steel line pipes nearby. A variety of cathodic protection testing technologies, including cathodic protection effectiveness testing, pipe-to-soil potential monitoring, in-pipe current monitoring and pipe corrosion monitoring, were applied along the whole pipeline to continuously test the pipe corrosion and cathodic protection parameters in the discharge process of grounding electrode. Then, the influence degree of the UHVDC in earth on the line pipes and the effectiveness of the corrosion control measures were investigated. And the following research results were obtained. First, in the process of discharge, corrosion protection measures (such as potentiostat and drainage zinc strip) play an active role in mitigating pipe corrosion, but corrosion still happens in some pipeline sections. Second, the discharge of grounding electrode of the line pipes in this area interferes the whole pipeline with the maximum current in the regional pipe 4.84 A, the maximum negative shift of pipe-to-soil potential -4 853 mV, the maximum positive shift +568 mV and the maximum corrosion rate of the pipe wall 0.049 mm/a, which are beyond the control target of the pipeline design. Third, the discharging of UHVDC grounding electrode has a greater impact on line pipes. In conclusion, the research results can provide data support for the compilation of testing and evaluation criteria on the influence degree of UHVDC in earth on buried steel line pipes.