The ecosystem damage baseline assessment is the foundation and premise of the environmental damage identification and evaluation. As the most widely used method in baseline assessment, reference site method can overcome the problem of lacking historical data. In the process of the assessment, choosing the different reference site can cause different reference conditions, which affect the degree of anthropogenic disturbance and the set value of damage baseline. At present, there is no universal discussion and evaluation about the reference condition of grassland ecosystem. In this paper, we proposed evaluation index of the reference site method in grassland ecosystem, categorized and filtered the samples in bibliography to form four conditions, and then chose the most accurate condition to determine the damage baseline. We took the typical grassland of Xilinhot in Xilingol League as the case study. The result showed that for natural reserve and experimental stations, it is appropriate to use minimally disturbed condition and historical condition as the baseline; for the areas where human implemented positive interference, it is appropriate to use the best attainable condition as the baseline; for the most other parts of grassland, it is appropriate to use the least disturbed condition as the baseline.