Abstract: The threshold of water exploitation index is a controlling indicator for water stress assessment, risk control and comprehensive management. Although the threshold which is usually set as 40% of the ratio of annual water withdrawals to annual available water resources has been widely used in water resources arrangement, its justification has not been fully explained. Furthermore, the threshold is always a constant and fails to reflect the uneven spatial and temporal distributions of water resources, so it is a challenge to implement the threshold in the practice of water resources management. According to the complementary relationship between the threshold of water exploitation index and ecological flow, a modified Tennant Method based on multilevel habitat conditions（MTMMHC Method）is adopted to estimate the ecological flows in Hanjiang River Basin on the basis of simulated runoff from SWAT Model. And dynamic thresholds of water exploitation index are determined by the complementary relationship. The results show that the MTMMHC Method can simultaneously consider the intra-annual and inter-annual variabilities of flow and reflect the difference of flow in wet, normal and dry years, and the thresholds of water exploitation index based on the MTMMHC Method in each sub-basin are different in Hanjiang River Basin. In addition, most of the thresholds of water exploitation index corresponding to the optimal ecological flows are less than 20% and are obviously lower than the so-called international standard of 40%. The proposed method is based on the complementary relationship between the water exploitation and the ecological flow, and it has taken the unevenly spatial and temporal distributions of water resources as a dynamic metric for water resources management.