We used a mechanics conceptual model to provide another perspective to understand the mechanical environment of the San Andreas Fault(SAF),and a possible mechanism that the principal stress state in the SAF is not only affected by remote tectonic stress but also by Poisson’s ratio.For a strike-slip fault like the SAF,we found that in the fault zone with Poisson’s ratio of[0.25,effective friction coefficient and the stress ratio(minimum principal stress/maximum principal stress)are less than 0.1and 0.8–1.0,respectively,corresponding to remote tectonic stress ratio of 0.36–1.0,and that the higher the Poisson’s ratio,the greater the principal stress rotates.For hydrostatic pore pressure and a received tectonic stress ratio of 0.5around the SAF,the model predicts that the SAF has a very high Poisson’s ratio(*0.45),which accommodates extremely low effective friction coefficient(0.09)and large stress ratio(0.84)or smaller shear stress(17 MPa). We used a mechanics conceptual model to provide another perspective to understand the mechanical environment of the San Andreas Fault (SAF), and a possible mechanism that the principal stress state in the SAF is not only affected by remote tectonic stress but also by Poisson’s ratio. For a strike-slip fault like the SAF, we found that in the fault zone with Poisson’s ratio of [0.25, effective friction coefficient and the stress ratio (minimum principal stress / maximum principal stress) are less than 0.1 and 0.8-1.0, respectively , corresponding to remote tectonic stress ratio of 0.36-1.0, and that the higher the Poisson’s ratio, the greater the principal stress was recorded. For hydrostatic pore pressure and a received tectonic stress ratio of 0.5 around the SAF, the model predicts that the the SAF has a very high Poisson’s ratio (* 0.45), which accommodates an extremely low effective friction coefficient (0.09) and a large stress ratio (0.84) or smaller shear stress (17 MPa).