In order to better understand euxinic evolution of the Ediacaran ocean in South China, a study of authigenic pyrite morphology and size distribution from the Doushantuo formation in deepwater area has been conducted. The result shows that most pyrites in the Doushantuo formation were formed below sediments/water interface during early diagenesis. They are generally euhedral or semieuhedral in shape, some also form framboids, but most (89%~96%) of them are larger than 10μm in diameter. These diagenetic pyrites may not reflect euxinic water condition. There are several intervals of syngenetic framboidal pyrites ( More than 96% with diameters < 10μm) occurring in middle and upper Doushantuo formation, which indicate euxinic condition. FePY/FeHR ratio in sediments as an important proxy alone for environment euxinia may be biased due to the superimposition of secondary pyrites generated within sediments during early diagenesis. Therefore it is better to use this proxy in combination with other methods, such as the analysis of pyrite morphology. Using these methods along with carbon and sulfur isotope analysis, the present study shows that the Ediacaran ocean in South China had experienced two episodic euxinic intervals during upper sedimentary cycle I and lower sedimentary cycle III of Doushantuo Formation. Euxinic water body may have expanded to slope facies area in extent during these intervals. The temporal and spatial evolution of the euxinic environments seemed to have been mainly controlled by the variation of ocean oxidation, sulfate concentration, bacterial sulfate reduction (BSR), sealevel fluctuation and their interactions. The known major fossil groups in this period largely occurred in the noneuxinic intervals, which might means that, in addition to ocean oxidation, the euxinic condition also played an important role on the biotic evolution in this period.