By using the single-link cluster method, precursor clustering features in time-space of the Tangshan and Datong earthquakes are studied. Time-space scanning to information entropy, average link length of the SLC frame and accumulation slide frequency of precursors of the two earthquakes is made. The results show that anomalies of the SLC frame before the earthquakes are very obvious, and can be divided into five periods. The anomalous magnitude is the largest from June, 1975 to June, 1976 (fourth period). The information entropy drops further, change of average link length is the same with that in the fourth period and accumulation slide frequency of the precursors increases one month before the earthquakes (fifth period).The change period of anomalies may be taken for the judging index of middle or short term prediction of earthquakes. Change process of information entropy from outside to area of coming earthquake epicenter may be used to judge the range of earthquake danger area.