Due to the limitations of both the graphic method and the rectangular method in determining the pore structure eigenvalues of rock, an integral method is proposed, in which the pore radius is integrated to determine the eigenvalues. In applying the integral method, the pore radius is first determined through a capillary pressure model. Then a capillary pressure power function is derived and presented in the double logarithmic coordinate system, and the model parameters are acquired using the least square method. The proposed model is validated through comparing the calculations with the data provided in Sami’s paper. A connection between rectangular method and integral method is identified, showing that the pore structure eigenvalue can be calculated using the integral method. Finally, comparison of the proposed method with other two methods indicates that the results calculated by the integral method are closer to the real values. This study can help understand the characteristics of rock micro-pore structure.