In order to improve the detection precision of fly ash carbon content using microwave method and fill the blank of fly ash electromagnetic parameter measurement in broadband, the coaxial transmission line method and coaxial probe method are used to study the frequency response characteristics of the S parameter and complex dielectric constant of fly ash with different carbon contents in the frequency range of 3~18 GHz in experiment, respectively in this paper. The experiment results show that in the frequency range of 3~18 GHz and carbon content of greater than 7%, there is a good linear relationship between S21 and carbon content, the fitness degree is greater than 0.96, and the measurement sensitivity is positively correlated with frequency. In the frequency range of 10~18 GHz, the carbon content and dielectric constant ε′ show a good linear relationship, the fitting degree is greater than 0.98. The relative uncertainty is below 4%. In addition, the reason for the poor regularity and repeatability of the S11 and loss factor measurement results are analyzed from the test uncertainty and measurement methods.