The research on the stress and strain of small-diameter steel pipe piles mainly focuses on landslide treatment, slope support, rescue and disaster relief projects, and less on karst areas. The bearing mechanism of small-diameter steel pipe piles in karst subgrade under inclined load was studied. By decomposing the stress-deformation problem of small-diameter steel pipe single piles under karst subgrade into soil layer section, karst cave section and embedded section, the m-method foundation assumption, Euler-Bernoulli beam theory and finite element simulation method were used to solve the deflection curve differential equations of different sections and the ultimate bearing capacity of single piles. Through finite element simulation, the theoretical calculation results and numerical simulation results were compared and analyzed. It is found that the error between the two is within 12%, which verifies the correctness of the theoretical model. It's pointed out that the pile side friction resistance has the most significant influence on the deflection curve of the steel pipe pile, and the ultimate bearing capacity of the single pile is mainly determined by the pile side friction resistance and the pile bottom resistance. According to the differential equation of the deflection curve, the bending moment, shear force, horizontal displacement and rotation angle at any point of the pile can be determined. The method proposed in this paper provides a theoretical basis for the design and construction of small and medium-sized steel pipe piles in karst subgrade, and has important engineering application value.