In order to determine the exploitable rate of river water resource by hydropower projects, we established a variable interval calculation method for river runoff of period distribution to calculate the river minimum ecological water demand and the maximum river torrent. Taking Lhasa River in Tibet, China as an example, the minimum ecological water demand and the maximum river torrent were calculated. The results show that the minimum ecological water demand and the maximum river torrent of Lhasa River are 29. 5~328. 3 m³/s and 95. 1~1 673. 4 m³/s, respectively. Compared with the constraints of minimum ecological water demand, the constraints of variable interval of river runoff decrease the water resources exploitable rate by the regulation development of water and electricity in Lhasa River from 60. 2% to 18. 7%. The author of this paper thinks that as for the seasonal rivers which have a significant characteristics of seasonal abundance and dry runoff, large-scale water projects should be constrained by the river variable interval during its balance to the runoff seasonal distribution, especially by the increasable interval of river runoff under the maximum torrent constraint in the dry season.