Abstract: In this paper the precursor anomalies of 51 moderate earthquakes in China from 1969 to 1984 are studied and their different and common properties are analysed. According to the reasults,the bases and methods to evaluate the mornitoring ability of seismic precursor observational networks in China are suggested. At the same time, the distribution map of the monitoring ability of the networks of the whole country is also given. Finally, the technical measures to promete the monitoring ability of seismic precursor observational networks are discussed.