Nine heavy metals (Cd, Co, Ni, Cu, Zn, Pb, Cs, As, Cr) of surface sediments collected from the Middle Part of the Yarlung Zangbo River were analyzed by BCR sequential extraction procedure. Based on speciation distributions of these metals, ecological risk of heavy metals was assessed with methods of risk assessment code (RAC) and sediment quality guideline quotient (SQG-Q). The results show that: (1) Large part of Cr and As belong to residual fraction(87%-96%). Ni, Cu, Co and Cs have similar distribution characteristics (non-residual part accounts for approximate 20%). Correspondingly, mass fraction of Cd exists mainly in acid soluble and reducible faction (65% ) , indicating high ecological risk level. (2) Based on calculation of RAC method, Cd is of high risk to the environment (37. 38%). Accordingly, Co, Ni, Cu and Zn are of low risk, and Pb, Cs, As and Cr pose extreme low risk to the environment. According to evaluation of heavy metal speciation distributions, the potential ecological risk of heavy metals is in the descending order of Cd > Co > Ni > Cu > Zn > Pb > Cs > As > Cr. (3) The SQG index of sediments is 0. 804, indicating that studied river sediment has a moderate potential biological toxicity effect. Given most of heavy metals exists in residual fraction, the sediments of this region pose a low ecological risk to the environment.