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Based on the Hermite-Gaussian expansion of the Lorentz distribution and the complex Gaussian expansion of the aperture function,an analytical expression of the Lorentz-Gauss vortex beam with one topological charge passing through a single slit is derived.By using the obtained analytical expressions,the properties of the Lorentz-Gauss vortex beam passing through a single slit are numerically demonstrated.According to the intensity distribution or the phase distribution of the Lorentz-Gauss vortex beam,one can judge whether the topological charge is positive or negative.The effects of the topological charge and three beam parameters on the orbital angular momentum density as well as the spiral spectra are systematically investigated respectively.The optimal choice for measuring the topological charge of the diffracted Lorentz-Gauss vortex beam is to make the single slit width wider than the waist of the Gaussian part.