Abstract: Along most small mountainous rivers, hydrological observation is very poor or completely lacking. It is impossible to estimate their ecological water demand using traditional hydrological methods that directly analyze river hydrological observation data. Xiaojin is a small river with a drainage area 33km2 located in Boluo county of Guangdong province, and along its mainstream seven small hydropower plants were developed in the past half century. In this work, a monthly water balance model with two parameters was adopted to recover the past flow processes in the Xiaojin River for the period of 1980-2011. Then, several traditional hydrological methods were applied to calculation of river ecological water demand. This provides a strategy for study of ecological water demands by the ungauged (or data-poor) mountainous regions. This paper describes a strategy of minimum water supplement based on water volume rather than flow rate, named sustainable static water areas for river ecology, which considers the life cycle characteristics of aquatic lives, their habitat conditions, and the benefits of hydropower stations on small mountainous rivers. By applying this conceptualization, river ecology and hydropower development can be well balanced in hydrological and hydraulic evaluation of minimum ecological water demands.