【Objective】 This study aimed to understand the soil stable organic carbon (SSOC) storage of natural grassland in the hinterland of the Qinghai-Tibet Plateau, and clarify its carbon fixation capacity and potential for increasing sinks. 【Method】The soil in the Three-River Source Region was divided according to regional space and three typical grassland types and the active layer soil was taken as the object. The soil total organic carbon (TOC) and Iron (aluminum)-bounded carbon [Fe(Al)-C] were analyzed, the relationship model was established with the geographical and climatic data, and the temporal and spatial variation law in recent 60 years (1961-2020) was retrieved. 【Result】The results show that: 1) The average content of Fe(Al)-C is 6.07 g·kg-1 and the average content of [Fe(Al)-C]/TOC is 16.87% in the Three-River Source Region, with obvious zonal characteristics. Also, the content in the central and eastern regions was significantly higher than that in the western region (P < 0.05); 2) The contents of Fe(Al)-C and [Fe(Al)-C]/TOC in the three types of grassland were 2.35 ~ 8.81 g·kg-1and 11.99%~20.52%, respectively, and the alpine shrub meadow and alpine meadow were significantly higher than those in alpine steppe (P < 0.05). Fe(Al)-C was positively correlated with TOC in the three typical natural grassland types (P < 0.01). 3) The results of analog- digital mapping showed that the distribution area change of Fe(Al)-C in the Three-River Source Region has changed in the recent 20 years (2001-2020). Compared with the past two periods; 1961-1980 and 1981-2000, the distribution area with its content >5.75 g·kg-1 and increased by 1.64% on average. 【Conclusion】From our analysis, it was deduced that whole area is in a state of carbon sink, and the C fixation potential of Fe(Al)-C in soils in the Three-River Source Region can not be overlooked.