Abstract: Some precursory anomalies of subsurface fluid before the Wenchuan 8.0 earthquake was extracted by combining strict qualitative and quantitative method. The data of water level and radon changes in Sichuan and Yunnan provinces had been analysed according to the idea of "identity" and the method of subordinate function. This paper gave an introduction and the preliminary results of characteristics of five typical examples which showed precursory anomalies of subsurface fluid. It is obvious that the trend anomaly of subsurface fluid appeared before Wenchuan earthquake, and that these anomalies are similar to those of other strong earthquakes in China.

Key words: Wenchuan 8.0 earthquake, Determination methods of anomalies, Medium-term subsurface fluid anomaly , Repeatability of anomaly