The growing season of alpine grassland is regarded as the vital indictors reflecting the vegetation growth, and is sensitive to regional climatic change in Northern Tibet. However, limited studies have identified the growing season variability across alpine grassland in Northern Tibet during 2001–2018. We investigated spatial-temporal variations in the growing season of alpine grassland in Northern Tibet, as determined by TIMESAT model employing MODIS NDVI data. Results indicate that, the start of growing season (SOS), end of growing season (EOS) and growing season length (GSL) for alpine grassland experienced delaying, advancing and shortening trends at a rate of 1.32, −2.06 and −3.38 days/10a, respectively, in the Northern Tibet. The variation of growing season in different alpine grassland types showed different changing tendencies. The SOS for alpine grassland was significantly positively correlated with the mean temperature and negatively correlated with the cumulative precipitation from January to May. The EOS for alpine grassland was positively correlated with temperature and precipitation in September. These results could contribute to the research on plant phenology and climate change from a different perspective.