Abstract: Knowledge of the in-situ stress underground is of crucial importance for drilling and reservoir development,the widespread use of wellbore imaging devices made it possible to interpret the local stress state more complete and economical.Breakout azimuths revealed by well logs are widely used to determine principle stress directions in traditional method,however strength anisotropic related to bedding planes may affect both breakout azimuths and widths.which can result in misleading in-situ stress interpretations.Based on weak-plane failure model,we analyzed the relationship between in-situ stress and breakout geometry(azimuth,width and depth).The results show that breakout azimuths are controlled by weak planes' azimuths,while steeply dipping bedding planes widen the breakout and make it deeper.For determined strata property,breakout geometry will change regularly with the increase of the principal stress ratio.A breakout observed in well WFSD-2 help introduce the method for constraining the maximum horizontal principle stress.