UHVDC transmission capacity is large, there will be a great impact on both sides of connecting ac system while one pole is outage，leading another pole in monopolar ground circuit operation mode. At this time the high amplitude dc current flowing into ground will have negative effects on the AC system and the metal parts nearby, producing DC magnetic biasing in converter transformers in operation and transformers nearby, threatening the safety operation of converter transformers and other transformers. Optimizations of the protection strategy in UHVDC including the converter differential protection，the grounding electrode line overload protection, the grounding electrode line unbalance protection and line fault restart logic is described in this paper, extremely shortening the operation time of the monopolar ground circuit operation mode after action of above protections, reducing influence of DC current flowing into the earth on the AC system，metal parts, converter transformers and other transformers. In order to improve the electrode line protection level, the grounding electrode line differential protection and supervision are added. These strategies have been applied in Hami-Zhengzhou UHVDC project and Linzhou-Shaoxin UHVDC project, the result of field test is effective and good.