Abstract: To explore seasonal variations in the potential nutrient supply and carrying capacity of grazing pastures in the Three-River Source Region, three adult Datong yaks equipped the permanent rumen fistula were chosen to provide rumen fluid. Analysis of this fluid was combined with measurement of grass yield to comprehensively determine the pastures’ carrying capacity and gas production, using general nutrient analysis and in vitro techniques. The results showed that the maximum yields of edible dry grass in summer, autumn, winter and spring were (123.83±17.88), (256.88±29.90) and (246.83±66.73) g/m² respectively. Maximum crude protein (CP) contents were (12.69±0.13)%, (10.54±1.22)% and (8.65±0.64)%, the contents gradually decreasing with grass growth. The maximum ether extract (EE) contents in summer, autumn, winter and spring were (2.95±0.10)%, (4.38±0.17)% and (3.74±0.70)% respectively. The change trend in neutral detergent soluble (NDS) content was consistent with CP, while neutral detergent fibre (NDF) and acid detergent fibre (ADF) contents varied negatively with CP. The pH value and NH₃-N concentration of solutions fermented in vitro were within normal ranges. The maxima of 48 h gas production and digestible energy (DE), metabolizable energy (ME) and organic matter degradation rate (OMD) (estimated by 24 h gas production) for summer pastures were (57.50±4.27) mL, (9.32±0.59) MJ/kg, (7.98±0.62) MJ/kg and (57.93±3.23)%; for autumn pastures (54.67±5.35) mL, (8.83±0.64) MJ/kg, (7.47±0.68) MJ/kg and (55.26±3.52)%; for winter and spring pastures (58.83±4.51) mL, (9.56±0.60) MJ/kg, (8.24±0.63) MJ/kg and (52.69±5.14)%. Without the provision of concentrates, according to grass and digestible crude protein (DCP) production results, the optimal yak carrying capacity of summer, autumn and winter and spring pastures was 7.05,19.51 and 2.47 SU/ha respectively. When supplementary concentrates were fed, maximum carrying capacity for the three seasons can reach to 14.85, 29.00 and 5.03 SU/ha (based on DCP production for summer pastures and ME production for autumn, winter and spring pastures). In conclusion, the forage yield and quality of grazing pastures varies significantly by season. By supplementary feeding, carrying capacity in summer, autumn, winter and spring can be increased by 1.1, 0.5 and 1.0 times, a result that would help to promote both local animal husbandry and ecological sustainability.