The data from 118 fixed precursor observation stations in north region of Qinghai - Xizang Plateau are selected for research. A method of synthetic anomaly index, including the time and spatial synthetic index, extracting from the data of precursor observation nets, is presented. Using the spatial synthetic index we can research the evolution characteristics of short-term precursor before moderate-strong earthquake in the region, and using the time one we can judge the occurring time for future earthquake. The problem of anomaly area drifting caused by the station not well-distribute is also solved. As the example, the change cause of the time and spatial synthetic anomaly index before Xinhai Ms6.6 earthquake on Sept. 12, 2000, which showed ＂ spreading→consentrating to the epicentre→earthquake→spreading again＂, is introduced.