A new method to calculate the refractive index of a capillary wall simply and accurately was proposed to reduce the effects of the spherical aberration and the depth of field on measurement results under a non-paraxial condition. Firstly, two kinds of standard liquids with known refractive indexes were filled into a capillary, respectively, and the distance between two focal positions were measured when the collimated light passed through the capillary. Then, by using the formula of the refractive index of capillary wall and the focal length, the refractive index of the wall-material could be obtained. The experimental results indicate that the measurement accuracy of refractive index is better than 0.003 when the refractive index of wall-material is less than 1.51 in the proper inner and outer diameters of a capillary. It is demonstrated when two kinds of low (high) refractive index liquids are chosen as standard liquids, the measurement accuracy can be improved. The introduced method has a simpler calculation process and its measurement precision of refractive index meets the requirements of general scientific research.