The transmission of 10-keV Cl^– ions through Al₂O₃ insulating nanocapillaries is studied both by experiment and simulation. The double-peak structure in the transmitted angular distribution is found to be the same as our previous result. The peak around the direction of the primary beam is caused mainly by the directly transmitted Cl^–, and the other peak around the tilt angle of Al₂O₃ nanocapillaries is mainly induced by Cl⁺ and Cl⁰. The intensity of transmitted Cl^– decreases with the tilt angle increasing, which is in accord with the geometrically allowed transmission. Beyond the geometrically allowed angle, the transmitted projectiles are mainly Cl⁺ ions and Cl⁰ atoms. The ratio of transmitted Cl⁺ ion to Cl⁰ atom drops as tilt angle increases, and it turns more obvious when the tilt angle is larger than the limit of the geometrical transmission. A detailed physics process was developed within Geometry and Tracking 4 (Geant4) to perform the trajectory simulation, in which the forces from the deposited charges and the image charges, the scattering from the surfaces as well as the charge exchange are taken into consideration. The transmissions at the tilt angle of 1.6^o are simulated for the cases without and with deposited charges of –100 e/capillary. For the deposition charge quantity of –100 e/capillary, the majority of the transmitted projectiles are mainly the directly transmitted Cl^– ions exiting to the direction of tilt angle, and the transmitted Cl⁰ and Cl⁺account for a very small portion. While for the case with no deposited charges, the simulation results agree well with the experimental results. The dependence of the scattering process on the tilt angle, which results in the different features in the transmitted projectiles, is studied in detail by the simulation. It is found that the transmitted Cl⁰ atoms exit through single to multiple scattering, and most of transmitted Cl⁰ atoms exit through single and double scattering, and are centered along the axis of nanocapillaries, while Cl⁺ ions mainly exit by single scattering, which results in the fact that the intensity of the transmitted Cl⁰ atoms drops slower than that of the transmitted Cl⁺ ions with the increase of the tilt angle, leading the ratio of the transmitted Cl⁺ to Cl⁰ to decrease as the tilt angle increases in experiment. Our results describe the physical mechanism of low-energy ions through insulating nanocapillaries in detail, i.e. how the scattering process dominates the final transmission. It is found that the transmission of the negative ions in the energy range above 10 keV is caused by the scattering and the charge exchange process.