Marine-continental transitional shale is highly susceptible to wellbore collapse due to its high pore pressure，strong water absorption and swelling tendency，pronounced bedding structure，and low tensile strength. Horizontal in-situ stress is a critical parameter in wellbore stability analysis. However，the complexity of geological conditions makes the accurate prediction of horizontal in-situ stress highly challenging. Existing studies often assume a circular wellbore，overlooking the possibility that the wellbore may naturally form an elliptical shape under non-uniform in-situ stress，thereby affecting prediction accuracy. An elliptical wellbore model was developed based on strain-softening characteristics and theoretical stress distribution models for the elastic and plastic regions was developed. The strain-softening behavior of shale was validated through triaxial compression tests，which served as the basis for deriving the stress distribution around both circular and elliptical wellbores. Furthermore，an elliptical wellbore model was derived established，and a horizontal in-situ stress prediction equation was proposed. Numerical calculations of stress distribution were implemented through programming，and finite element simulations were conducted to validate the stress distribution at different aspect ratios，showing good agreement with theoretical results. The results indicate that the elliptical wellbore model more accurately reflects wellbore stress distribution. As the aspect ratio increases，the extreme values of radial and tangential stresses increase significantly，with notable stress concentrations occurring particularly at 180° and 360°. Based on the derived elliptical wellbore model，the predicted horizontal in-situ stress shows an accuracy of over 89% compared to field data，verifying the model’s reliability. The research results overcome the limitations of the traditional circular wellbore assumption，providing a more accurate prediction method for wellbore stability analysis in complex geological environments. It also offers critical theoretical guidance for wellbore stability control and drilling engineering design in field applications.