Abstract: This paper deals with instable temperature field caused by thermal events and demonstrates that abnormal temperature distributions and heat flow values depend on the event parameters such as intensity,life-span and time it occurred.Comparing some theoretical results with observation data,it is found that the passive block and the active region are different in thermal processes and that the differential depth of low-velocity layers/low-resistivity layer between the crust and upper mantle is correlative with thermal-activation processes.The differential depth between the two layers ranges from 40km to 60km for the North China plain,suggesting a therml event of about 50—70m.y., and a duration of about 30—40m.y.,while for the Yin Chuan basin,it ranges from 36km to 40km,suggesting a thermal event of 30—40 m.y..Generally,the effects of thermal events which occurred more than 100m.y.ago can be neglected and the temperature field can be regarded as the stable.The method based on stable temperature field to determine structures of crust and upper mantle is only suitable to passive blocks.