Wenchuan Fault Scientific Drilling（WFSD）is the quickest earthquake fault drilling project in the world.When borehole caving and hydraulic fracturing methods can hardly be implemented

the corebased method

i.e.the inelastic strain recovery method

is first used to determine three-dimensional insitu stress in deep drilling wells.Based on the theory of inelastic strain recovery of viscoelastic body and core testing principle

the inelastic deformation of deep hole core under three-dimensional in-situ stress is simulated

and the magnitudes and directions of the three principal stresses are calculated under the assumption of isotropy.In the laboratory of WFSD project site

the developed testing systems are used to test the normal strains with six independent directions of inelastic recovery strain of the deep drill core

and to determine the magnitudes and directions of three-dimensional in-situ stresses at the measure point.In order to validate the inelastic strain recovery method

the test results of the No.1 hole of WFSD project have been analyzed

which are compared with those obtained by focal mechanism solutions and other stress measurement method.The results show that the inelastic strain recovery method is reasonable and reliable

especially under complicated geological conditions when other stress testing methods can not be used.The proposed method can obtain reliable data with good adaptability

which provides a new way for carrying out in-situ stress measurement by using the core method under the deep drilling conditions.