Abstract: Current researches on river ecological water demand in China more focus on arid area or ecologically degraded rivers rather than the southern rivers, not to mention study on medium rivers in hilly region of Guangdong Province. Guangdong is rich in water resources, but due to intensified development of water resources and increased water pollution, ecological degradation of the rivers has become an urgent problem. According to the runoff and cross section data at Shuangqiao hydrological station on Siheshui River from1980 to 2010, six methods are employed to estimate and analyze the ecological water demand of Siheshui River. The results show: the results estimated by NGPRP method, the minimum monthly average stream flow and monthly minimum ecological stream flow are relatively ideal, but using the same ecological water flow for the whole year does not meet the natural law because that water amount in flood season is more than that in non-flood season and biological breeding habitat requirements in flood season are higher than biological overwintering habitat requirements in non-flood season. If the ecological water demand in flood season and non-flood season are calculated separately according to frequency order, the result can meet the natural law of river runoff and biological breeding. The research can provide ideas and methods for the estimation of ecological water demand in Guangdong hilly area.