In order to improve the quality of laser induced breakdown spectroscopy, we used a time-resolved spectrometer to collect steel target plasma emission spectra. The relationship between the intensity of spectral lines and the delay time was discussed. It demonstrates that both the spectral intensity and background intensity decay exponentially with the delay time. Spectral intensity of atoms decreases faster in the first 4 μs but the atoms′ spectral lines exist for a longer time. However, ions′ spectral lines exist for shorter. Acquisition delay has different influence on signal to noise ratios of different spectral lines. The best delays of Mn I 403.08 nm ,Cr I 428.97 nm, Cr Ⅱ 458.82 nm, Fe I 430.79 nm and Fe Ⅱ 503.57 nm spectra obtained are 8, 2, 0, 2, 4 μs, respectively. The double lines method and the Boltzmann plot method were used to calculate the plasma temperature， and the Saha-Boltzmann equation was used to calculate the electron density. It is verified that the spectral signal meets the local thermal equilibrium state in the acquisition time range of 0-10 μs.