We report the temporal and spatial C-isotopic variations of carbonate (δ¹³C_carb) and organic carbon (δ¹³C_org) spanning the Permian-Triassic transition at the Xiakou and Chaotian sections in South China. The C-isotope data from the two sections show two episodes of δ¹³C_carb decrease. However, in contrast to most of the C-isotopic records of the Phanerozoic, the changes in δ¹³C_org are not paralleled by those of δ¹³C_carb: rather, they show a gradual increase followed by a consistent negative excursion. Accordingly, the C-isotopic data exhibit two episodes of decreased Δ¹³C_carb-org (Δ¹³C_carb-org = δ¹³C_carb – δ¹³C_org). A dual-reservoir model was used to study the main factors responsible for the C-isotopic excursions. We show that two episodes of decrease in δ¹³C_carb and Δ¹³C_carb-org may be best explained by the remineralization of organic carbon and the proliferation of green sulfur bacteria, which may have been driven by the episodic incursion of sulfidic waters. The temporal and spatial variations of δ¹³C_carb and δ¹³C_org reveal two episodic incursions of sulfidic waters into shallow water areas with resulting photic zone euxinia. The first incursion of sulfidic waters occurred before the end-Permian mass extinction (EPME) event, while the second incursion occurred across the EPME event. The modeling results show that the amount of ¹³C-depleted carbon input from remineralization across the EPME is ~1.5-times that before the EPME, providing quantitative evidence for the temporal changes in episodic euxinia. This conclusion is strengthened by the observed δ¹³C depth gradient between the two sections. Our results provide quantitative constraints on carbon cycling in the Permian-Triassic oceans and suggest that the expanded euxinia may have crossed a threshold that led to the most severe biotic crisis in Earth's history.