Abstract: The load transfer law of the semi articulated flexible drilling tool directly affects the drilling in ultra short radius horizontal wells. In this paper, firstly, the element stiffness equation is deduced and the bending moment transfer rule of the semi articulated flexible drill is obtained. Then, combining the theoretical analysis and the numerical simulation, the semi articulated flexible drill rod model is verified. Finally, the semi articulated flexible drill mechanical model is established in the build section and the load transfer under different borehole curvature radius conditions is discussed. The analysis results show that with the same well depth, with the increase of the radius of curvature, the bottom hole torque value increases, when the semi articulated flexible drill rod is not locked, the semi articulated flexible drill rod does not transfer the bending moment and increase the large fluctuate torque at the bottom hole . The research results provide a theoretical basis for the design of flexible drilling tools.