As a physical index to express the information on topography, topographic index is a very important parameter for many physically based hydrological models. This article analyzes the topographic index based on a small tested loess watershed. The topographic index is derived f rom DEMs of the watershed by using the single flow direction method. Result s show that the spatial dist ribution of the topographic index has a high correlation with terrain dist ribution. Average topographic index variance has an exponential relationship with average slope variance.Frequency dist ribution of the topographic index shows a regular change with changed topography. The analysis of the topographic index variance can help us study catchment topography and hydrological similarity.