Abstract: To address the issue of low carbon content in fly ash during the actual operation of power plants, a calculation model for carbon content applicable to low-carbon-content ash samples was proposed. In order to enhance the measurement accuracy of S parameter, an error model was introduced for calibration, and relevant experiments were designed to validate the accuracy of this model. To achieve the benefits of high precision, short measurement cycles, and good flexibility in fly ash carbon content detection, a fly ash carbon detection system was developed using radio frequency devices and GNU Radio software, realizing functions such as ash sample calibration, S parameter calibration, ash level measurement, and carbon content measurement. To verify the measurement accuracy of the system, comparative experiments were designed. Results demonstrate that the calibration of S parameter through the error model can effectively improve measurement accuracy. Based on this, through the comparison between the measured ash level and the theoretical ash level value, it is revealed that the deviation between the measured result and the theoretical value does not exceed 5 mm. When comparing the measured carbon content with the standard carbon content, the results indicate that the absolute error between the two is no more than 0.18 percentage points.