Based on the interfacial-fluid-thickness method,a mathematical model of the high gradient threshold of refractive index was established and the aero optical propagation effects of high gradient region in hypersonic flow was simulated.A gradient threshold using its harmonic mean was proposed.The density fields of hypersonic turbulence obtained from computational fluid dynamics were used to analyse refractive gradient magnitudes and aero optical distortions.Refractive index gradients whose absolute values were below the threshold were set to zero and the new refractive index field was reconstructed.Using the ray tracing algorithm of variable index-of-refraction,aero optical distortions of the original and reconstructed refractive index fields of different cross-sections,different flow conditions were simulated numerically and analyzed synthetically.The comparisons revealed that,using the threshold,the two dimensional correlation coefficients of the original and the reconstructed index-of-refraction fields were high to above 0.9,the relative error of the root mean square of optical path difference was not more than ±5%.Through the numerical simulations,the effectiveness and adaptability of the threshold proposed was validated and it also showed that the high index-of-refractive gradient region in hypersonic turbulent flow was the main reason in aero optical distortions.