In this paper, comprehensive judgment on each anomaly of precursory anomalies of earthquakes is made by ideas of the "expert system", and probability of earthquake occurrcnce is calculated by means of the most likely time of earthquake occurrence for cach anomaly. The relationship between system entropy and field and s0urce precussor of earthquake is analysed by means of infomation entropy for groups of all sorts of precursory anomalies of certhquakes. In this paper, distribution character of time and space of in formation entropy for groups of all sorts of precursory anomalies of carthquakes before Tangshan earthquake (Ms7.8) in 1976 is analysed. Then the large area centred at Tangshan is divided into three parts: the near, middle and far areas. The evolution values of time of in fomation entropy for groups of all sorts of precursory anomalies of earthquakes in the three areas are respectively calculated, and the character of field and source precussory for this inforation entropy before earthquake in analyzed the infomation cntropy in the near area around the earthquake center.is much lower than in the middle and far areas.