Quantum impedance Lorentz oscillator (QILO) is a newly established model for quantizing Lorentz oscillators based on Bohr-Somerfeld theory and quantum mechanical selection rules. Based on this model, the second harmonic characteristics of Li₂SnTeO₆, CsClO₃, and Na₂Nb₄O₁₁ optical crystals were analyzed and numerically simulated, and a method for estimating the second-order nonlinear optical coefficients of crystals was proposed. First, we calculated the effective quantum numbers before and after the atomic transition according to the peak frequency and full width at half maximum of the optical crystals. Then, considering the second nonlinear effective parameter of QILO, we inferred and determined the second-order polarizability of the three crystals as a function of wavelength. As a result, the second harmonic generation coefficients of the three crystals at 532 nm were 0.17 pm/V, 0.69 pm/V, and 1.17 pm/V, respectively, which agree well with those from the first principle. The results show that the second-order electric susceptibility based on QILO model is helpful to analyze and improve the efficiency of sum frequency, difference frequency, and frequency multiplication of materials, and the method is simple, calculation time is less, and calculation efficiency is high.