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The Pailugou watershed of the Qilian Mountains in the arid region of northeast China was selected for research to analyze the species, height, and biomass in typical mountain grassland with an altitude of 2,700-3,000 m and measure soil moisture, so as to explore the seasonal characteristics of grassland biomass as altitude increases and the relationship between grassland biomass and soil moisture. The results showed that: (1) with a mean value of 135.36 g/m², grassland aboveground biomass showed a unimodal distribution from rise to decline as altitude increased, and reached its maximum of 176.79 ± 28.37 g/m² at an altitude of 2,900 m; with a mean value of 946.13 g/m², grassland underground biomass showed an uptrend as altitude increased, and reached its maximum of 1,301.19 ± 68.24 g/m² at an altitude of 3,000 m; (2) the difference between aboveground biomass and underground biomass in the grassland at different altitudes was significant; (3) values of root shoot ratio of the arid mountain grassland varied between 4.14-11.95; (4) values of soil moisture content of the arid mountain grassland varied between 9.23%-31.31%; (5) there was a positive correlation between aboveground biomass and underground biomass and the mean soil moisture content (p < 0.05), with a correlation coefficient of 0.778 4 and 0.784 3 respectively, soil layers with different moisture content made different contributions to grassland biomass, and moisture content in the soil layer where over-60-cm root systems were located was of significance to grassland biomass. [ABSTRACT FROM AUTHOR]