By using daily mean temperature and daily precipitation of Lasa, Rikaze, Zedang and Jiangzi in the middle course of Yarlung Zangbo River valley in 1971-2010, the variation of its climatic growing season was analyzed. The result shows: (1) The climatic growing season lengthened by 5~8 d·(10a)~(-1) obviously, the climatic growing season of limit temperature≥0℃ and ≥10℃ had increased by 30 d,limit temperature≥5℃ had increased by 20 d; with limit temperature≥10℃ being considered, the winter season shortened while the summer season lengthened. (2) The trend was significant of beginning dates advanced, ending dates deferred, lasting days and active accumulated temperature increased of three critical temperature; precipitation and ≥5 mm precipitation days of limit temperature≥10℃ was increasing by 19 mm·(10a)~(-1) and 1.5 d·(10a)~(-1), and confirmed a mutation in the end of 1980s, indicating the precipitation and temperature conditions of limit temperature≥10℃ was changing to warm-humid significantly. (3) The interannual variability of ending dates was relatively stable, and other elements was unstable; The decadal variability of the beginning and ending dates, lasting days and all accumulated temperature keep step changes, but other elements presented fluctuation changes, the minimum appeared in 1980s. (4) The variation trend of ending date, lasting days and accumulated temperature of limit temperature≥10℃, which can be considered mutation in the latter of 1990s, indicating the largest contribution to the extension of the summer season came from the significant delay trend of limit temperature≥10℃.