In order to determine the optimal detection time of laser-induced breakdown spectroscopy(LIBS), time-resolved LIBS technique has been adopted to study the decay characteristics of peak intensity and spectral intensity of four elements Fe, Pb, Ca and Mg contained in the sample. The extreme value of ratio of peak intensity of characteristic lines and spectral intensity of continuous radiation is calculated, and the calculation value of time corresponding to the optimal signal-to-background ratio (SBR) of characteristic lines is obtained. The peak intensity of characteristic lines and spectral intensity of continuous radiation both decay in exponential form, the decay time of four characteristic lines are 1.42, 1.95, 1.69, 1.65 μs, and the corresponding decay time of continuous radiation are 0.85, 0.93, 0.89, 0.87 μs respectively. The calculation value of corresponding time of optimal SBR of four characteristic lines are 1.43, 1.77, 1.62, 1.80 μs, and experimental measurement value of corresponding time of optimal SBR are 1.5, 2.0, 1.5, 1.5 μs. They have a good consistency. The corresponding time of optimal SBR can be predicted by evolution law of peak intensity of characteristic lines and spectral intensity of continuous radiation. The results indicate that the optimal detection time of LIBS is closely related to decay law of continuous radiation and characteristic lines, and the corresponding time of optimal SBR of spectral lines can be predicted by evaluating extreme value of the ratio of spectral intensity. The method provides the basis for selecting the optimal time of LIBS detection and analysis.