Fine root production and turnover is a critical component of ecosystem nutrient and carbon cycling and is also a sink for plant primary productivity. Poplar is a common-introduced protective plantation in the Yalung Zangbo River valleys of south Tibet Autonomous Region (TAR). However, little information has been concerned on the growth and productivity of this forest, especially for the belowground fine root turnover. The 20-year-old poplar plantation with 780 trees·hm-2 was sampled in Taktse county, in the lower reaches of Lhasa River valley. We measured the dynamics of fine root (with diameter < 2.0 mm) by soil core sampling every 10 cm layer to 50 cm depth in the growing season of 2004, May through October. Fine root samples in nylon bags were buried October, 2003 and collected in June and October of year 2004, 2005 and October of year 2006. Decomposition was evaluated by loss rates of fine root mass. 80% of the poplar fine root biomass was dominated in the 0-30 cm soil depths, with much of fine root mass distributed within 100 cm of trunk. The average biomass of fine root was 2.576 t·hm-2 in the growing period, and dead fine root of that was 1.566 t·hm-2. Net primary productivity of the fine root was 3.030 t·hm-2a-1, and turnover rate was 1.18 times per year. But the average decomposition efficiency (k) ranged from 0.0007-0.0008 due to >0℃ accumulated temperature was low. All of these characters favor the poplar trees to adapt the environment in the river valley of south TAR.