Internal standard method is one of the most popular quantitative analytical methods for quantitative analysis of laser induced breakdown spectroscopy (LIBS). In order to improve the precision of quantitative analysis, the rule of relative fluctuation characteristic of spectral line intensity ratio between analysisline and excitation line,energy differential (ΔE), and wavelength difference (Δλ) has been studied. Under thermodynamic equilibrium conditions, the mathematical model considers the electron energy level difference of atomic emission spectrum generated from high level to low level, plasma temperature, partition function, and the factors that influence the intensity of plasma, the influence of excitation energy differential on relative fluctuation of spectral line intensity has been studied. Under the condition of -2 eV<ΔE<2 eV and plasma temperature range in 3 000~15 000 K, the trend of spectral line intensity changes with ΔE and T is: When ΔE gets larger, the spectral line intensity increases, at the situation of ΔE=2 eV, T=3 000 K, spectrum intensity ratio is maximum. Further, relative fluctuatesof spectrum line intensity is sensitive to ΔE and T, and relative fluctuate can be smaller when ΔE tends to be zero. T has little influence on the relative fluctuation of spectral line intensity. At the situation T=10 000 K, relative fluctuates under the situation of ΔE<0 is smaller than ΔE>0, so ΔE spectrum line pairs with ΔE<0 should be chosen. With theoretical analysis, it has been discovered that if |ΔE| is close to zero, the relative fluctuation of spectral line intensity is smaller. In experiment, the Nd∶YAG pulsed laser was used to breakdown samples works with wavelength of 1 064 nm, pulse energy is 85 mJ, repeat frequency is 1 Hz, and pulse width is 13 ns. Andor company Mechelle 5000 spectrometer is used together with Andor New iStar model ICCD for spectrum gathering with working wavelength of 200~975 nm, optical resolution is better than 0.05 nm.The laser induced plasma spectra is verified by iron-based alloy. In the experiment, Fe is internal standard element, Cr/Mn are analysis elements. In this paper, the spectrum line with transition probability more than 106 of the NIST spectrum line library is chosen, and the non-resonant line with similarity resonance energy difference is compared and analyzed. Results show that the method of choose spectrum line with similar excitation energy level or similar wavelength has some limitations. For Cr and Fe, |ΔE| at 0.14 and 1.51 eV for the spectral line intensity of relative standard deviation (RSD) were 6.7% and 4.6% respectively, and theoretical value and practical value of the spectral line intensity were 1.14 and 0.59; RSD were 6.3% and 4.4% with |Δλ| of 11.7 nm and 50.8 nm, respectively. Difference between theoretical and practical values of the spectral line intensity was 1.69 and 0.62. Analysis showed that, influence of stimulated energy level difference of Cr/Mn is obvious than wavelength difference.To analysis element Cr/Mn, when internal standard element is Fe, the absolute wavelength value increases RSD decreases. Within a large scope of 1.50 eV and 90 nm spectrum range, larger |ΔE| can get smaller relative fluctuation characteristic of spectral line intensity ratio, and maximum RSD of Cr and Fe is 2.06%; large |Δλ| can get smaller relative fluctuation characteristic of spectral line intensity ratio, and maximum RSD of Cr and Fe is 1.35%. It can be concluded from the experimental results that, when the spectrum line is selected, the spectral line principle with similar excitation energy level and wavelength has certain limitations. If |ΔE| or |Δλ| is larger, RSD will be smaller. The spectral line intensity ratio which has similar theoretical and practical value can be a spectral line selection basis. Additionally, if theoretical value and actual value of spectral line intensity ratio are more similar, relative fluctuation of spectral lines intensity can be reduced.