MgO is one of the main substances which are required to be accurately determinated in cement. The plasma spectra of ordinary silicate cement obtained by laser-induced breakdown spectroscopy (LIBS) are analyzed. Mg I 517.2 nm characteristic line is selected as the analysis line. By measuring the spectral intensities of the signals and the corresponding signal-to-noise ratios with different laser energies. It′s found that the optimal energy is 40 mJ. In order to reduce the error, the ratio of the spectral line intensity of Mg 517.2 nm and the spectral integral from 516 nm to 520 nm is used as internal standard to establish the MgO mass fraction calibration curve. The carrelation coefficient of calibration curve is 0.9959. Cycle inversion method is used to detect the accuracy of MgO quantitative analysis. The maximum relative error and the mean relative error are 5.9% and 2.48%. The detection limit of MgO mass fraction in cement obtained by LIBS is 0.51%.