Flow prediction in ungauged basins (PUB) is as important as it is difficult. PUB is a major unsolved challenge in scientific and engineering hydrology. So far, empirical and stochastic methods have mainly been used for this purpose. Many small catchments in humid mountainous areas of China are with little or no hydrological data. The principal method of flood forecasting in this area is the parameter estimation of the hydrological model. Xin'anjiang Model was used to predict flow in this study, and Xin'anjiang Model parameters were estimated based on the physical meaning of them and regionalization. Regression and physical similarity approach were included in regionalization methods in this study. The 29 small basins in mountainous areas in southern Anhui Province were selected for gauged catchments, and three small basins in mountainous regions in western Hubei Province were treated as ungauged catchments. The main goal of this approach was to research whether regionalization and other methods could effectively estimate model parameters when the spatial location of the gauged catchments and the ungauged catchments were far away, but the physical conditions were similar. It is shown that:(1) Xin'anjiang Model has high accuracy in flow prediction of study areas. (2) Even if the space distance of gauged catchments and ungauged catchments is far, regionalization and physical-based estimation method can also reduce the efficiency loss caused by parameter estimation to some extent. (3) With the increase in the number of donor catchments, the efficiency of the physical similarity approach is reduced. (4) The optimal parameter estimation scheme in the study area is the ingenious combination of physical similarity approach (only one donor catchment), regression approach and physical-based estimation method. A feasible parameter estimation scheme of the Xin'anjiang Model is proposed in this study, which provides guidance for flow forecasting in small ungauged catchments in humid mountainous areas among the middle and lower reaches of the Yangtze River.