Abstract: Muddy shale is ductile rock,the initiation angle of muddy shale formation was researched to enhance the effect of hydraulic fracturing.In this work,an analytic model for plastic core region and fracture initiation of Ⅰ-Ⅱ mixed mode hydraulic fractures was proposed base on Plastic Mechanics and superposition principle.Based on the model,the plastic core region of fracture was calculated considering the fracture and maximum horizontal principal stress inclination angle of 0°,30°,45° and 60°.The calculated initiation angles were compared with the initiation angles by adopting maximum tangential stress criterion.Results showed that when inclination angles were 0°,30°,45° and 60°,the regime of plastic core regions is 0-0.18,0-0.45,0-0.45 and 0-0.18 times that of half-length of fracture respectively.When the inclination was 0°,the initiation angle was 180°,and the initiation angle decreased to 0° when the inclination angle was 90°.Given the same inclination angle and pressure,the fracture initiation angle of the new model was 0°-20° larger than the initiation angle based on maximum tangential stress criterion.With the same inclination angle,the initiation angle under 55 MPa was 0°-40° larger than that under 45 MPa.The research results showed that Plastic Mechanics was a feasible method for researching initiation of ductile formation,and the results could provide theoretical reference to the optimization of perforation angle and fracturing pressure of horizontal well multistage fracturing in muddy shale formation.