Haxilegen Glacier No.51 (43.731°N, 84.391°E; CN5Y741C0051) is located in the Kuytun river basin, Erenharbirga range, eastern Tien Shan. This study presents the annual mass balance of Haxilegen Glacier No.51 for 7 hydrological years and uses a temperature-index and an accumulation model to reconstruct the annual mass balance from 1999 to 2015. The model is calibrated against annual altitudinal mass-balance measurements and then applied to the period with no measurements. We find an accumulated mass balance of −6.06 ± 0.88 m w.e.a−1 over the period of 16 hydrological years, with an average annual value of −0.32 ± 0.22 m w.e.a−1. The mean glacier-wide annual, summer and winter balances for 1999 to 2015 are −0.37, −0.54 and 0.16 ± 0.22 m w.e.a−1, respectively, with a high correlation coefficient (r = 0.95, p < 0.001) between annual balance and summer balance. The calculated mass-balance sensitivity of the glacier to temperature is −0.51 m w.e.a−1 °C−1 and to precipitation is 0.08 m w.e.a−1 for a 10% increase. The sensitivity of seasonal mass balance indicates that temperature during the melt season (May–August) and annual precipitation are the major contributors to mass-balance fluctuation.