Calculated by standardizing the difference between potential evapotranspiration and precipitation, the standardized precipitation evapotranspiration index (SPEI) shows the drought degree deviation from normal conditions. As an ideal indicator for analyzing the drought evolution trend,SPEI is widely used in drought assessment,water resources management and other fields. The characteristics of the temperature,precipitation, temporal distribution and variation of the temperature,precipitation, slight,moderate and severe drought frequencies and their trends in growing season of grasslands were analyzed using the SPEI method based on the monthly mean air temperature and monthly precipitation at 8 meteorological stations over the Tianshan Mountains in Xinjiang during the period from 1961 to 2015. Results showed that the annual temperature in the study area was in a significant increase trend in recent 55 years. The precipitation was generally in a slight increase trend with a larger inter-annual variability,but the change trend was not significant. The numbers of drought events at monthly, seasonal, semi-annual and annual scales in the Tianshan Mountains were 42,25,16 and 7 respectively. The frequencies of slight,moderate and severe drought in growing season of grasslands were 37.5%,7.1% and 1.8% respectively. In recent 55 years, the climate over the grasslands in the Tianshan Mountains in growing season of grasslands became dry,and the annual degree of drought changed dramatically,which affected significantly the growth of the grasslands.