Abstract: Aboveground biomass (AGB) is a key indicator of grassland productivity and ecosystem functioning. Understanding the mechanisms through which environmental factors regulate AGB is essential for maintaining ecosystem functioning and ecological security. Based on an extensive field survey across the northern slope of the Tianshan Mountains, China, this study integrated threshold regression and piecewise structural equation modeling (SEM) to quantify the direct and indirect effects of biotic and abiotic factors on grassland AGB and to identify threshold-dependent shifts in these mechanisms. Results showed significant differences in AGB among grassland types, with meadow grasslands showing the highest AGB (191.32 g/m²), followed by steppe grasslands (85.24 g/m²) and desert grasslands (68.20 g/m²). Among biotic factors, vegetation height and coverage were reliable indicators of AGB. Vegetation height was most strongly correlated with AGB in meadow grasslands (r = 0.54), whereas vegetation coverage was dominant correlate in steppe and desert grasslands (r = 0.44 and r = 0.37, respectively). Climatic effects on AGB were nonlinear and threshold-dependent. In meadow grasslands, both precipitation and temperature promoted AGB growth below their thresholds of 263.7 mm and 6.5 °C, respectively; however, precipitation exceeded 263.7 mm, its direct effect reversed to significant inhibition (path coefficient: −0.25), while temperature remained the main positive driver (path coefficient: 0.45). In steppe grasslands, precipitation was the dominant driver and had a positive linear effect on AGB, whereas the positive effect of temperature was limited to values below 4.8°C. In desert grasslands, precipitation also dominanted AGB variation, while the effect of temperature shifted from positive to negative at 7.3 °C. Across all grassland types, climatic factors additionally affected AGB indirectly through their influences on soil nutrients and organic matter, which further regulated biotic attributes. These results provide new insightss into threshold dependent biomass regulation and offer a scientific basis for adaptive grassland management in arid regions.