In view of the calculation of ecological water demand in the river course, taking the downstream river channel of Chengbihe Reservoir in Guangxi as an example, three different hydrological calculation methods were used to estimate the ecological water demand, and the traditional Tennant method and the monthly guaranteed frequency method have been improved. Considering the shortcomings of the traditional Tennant method, the water use period is redivided, and the correction coefficient is introduced to revise the flow percentage to make it more suitable for the river. In addition, the monthly guaranteed frequency method has been improved in two aspects, so that the calculation results are more reasonable and reliable. The results show that the three methods all can be applied to the river ecological water requirement calculation. In normal years (P= 50 %), using the modified Tennant method and the improved monthly guaranteed frequency method to calculate the suitable water requirement of river, and the water demand are respectively 4.99×108 m3, 3.80×108 m3.In addition, by using the during the year distribution method we obtained results for 3.17×108 m3, which is relatively small compared to the above two methods, but within the reasonable limits, and still has some significance for extension use.