Abstract: Based on probabilistic preprocessing,The observation values for the observed items of subsurface fluid in the joint area of Hebei,Shandong and Henan province have been transformed into probability and time sequence.The synthetic information entropy of the information source system formed by subsurface fluid in the area is calculated.The relation between the variation of the synthetic information entropy and Heze M s5.9 earthquake (1983) and Renxian M_s5.0 earthquake (Hebei province,1985)is analyzed.The results show:there are two times obvious entropy decreasing before the two events and the later is more distinct than the former.The two events both occurred at the low entropy moment.The entropy recovered after the shocks.This process reflects maybe the ordinal change of the entropy decreasing in the course of earthquake development and occurrence for a subsurface fluid information system.Study shows 0.45 is the precursory anomalies' alarm line for an earthquake in the area,and the closer to an earthquake,the larger the entropy decreasing,the greater the magnitude of the earthquake.