To verify the ability of LIBS to measure the change of carbon content in fly ash, in this study, 24 samples with different carbon content were obtained by mixing pulverized coal and fly ash in a certain proportion. Spectra of these samples from LIBS were studied in different delay time. The emission intensity ratio of IMg1/IMg2 decreases with the increase of delay time. The emission intensity ratio of fly ash samples is obviously higher than that of other samples mixed with pulverized coal. The emission intensity ratio of IAl/ISi1 decreases with the increase of IMg1/IMg2. The addition of pulverized coal in the fly ash sample has a significant influence on the temperature change of plasma. By introducing the method of multivariate analysis, the ratios of spectral lines intensity of elements such as C, Al, Mg , Ca ,Fe to that of Si element were calculated as the independent variable of multivariate regression analysis, and the concentration of carbon element was taken as dependent variable, and the regression equation under different delay time was established. The results showed that the errors between the predicted values and the reference values based on the regression model were within 9%.