Each pole of ultra-high voltage direct current（UHVDC）transmission system has two valve groups structured in series，both coordination and interrelated factors is required between same-pole two valve groups.The control and protection logic and staggered affect of secondary circuit may bring about great risk for commission and maintenance work in the field.This paper focuses on the analysis of interacting mechanism between same-pole two valve groups and interacting influence of control protection logic and secondary circuit，which have met in Yunnan-Guangdong UHVDC transmission project.The factors of mutual influence and the potential security risk of same-pole two valve groups under particular operation modes for field work are described in detail.Several reasonable suggestions are given for safe work，which may be a reference to prevent the forced outage of operating valve groups due to imperfect technical measures.