With the rapid development of West-to-East power transmission and high voltage direct current (HVDC) technique, the impact of HVDC transmission line on oil-gas pipeline becomes increasingly severe. Researches have shown that the grounding current at DC grounding electrode of HVDC transmission line will generate interference potential on pipelines which leads to operational risks. To guarantee safe operation of oil-gas pipeline, this paper proposes a novel coordination control strategy of HVDC monopole block with grounding current constraint. Aiming at minimizing the change of overall generation output and adding fast-decreasing grounding current as constraint, the HVDC monopole block situation is modelled as non-linear programming and solved by infeasible interior point algorithm. The proposed method is verified on HVDC models of China Southern Power Grid in online open-loop circumstances and effectiveness of the algorithm is demonstrated.