To further improve the accuracy of the microwave waveguide method for detecting the unburned carbon content of fly ash, a method is proposed to calculate the ash level and relative group refractive index of fly ash in the waveguide. Through time domain transformation, the time of microwave reaching the interface of different media is achieved. Then, by combining with the transmission speed of microwave in different media in the waveguide, the reflection of microwave on the interface of different media in the waveguide is obtained. This paper uses a rectangular waveguide with a specification of 8 mm×15 mm×100 mm. Under the 12. 4-18 GHz frequency band, the above method tests the level and relative group refractive index of a series of ash samples with a mass of 0-12 g and a carbon content of 1% -10% . Results show that there is a critical point in the measurement process. When the actual ash level in the waveguide is larger than 47. 3 mm, the error between the measured level and the actual gray level is basically within ±2 mm. And the relative group refractive index of the ash sample in the waveguide is positively correlated with the carbon content of the fly ash. It does not vary with the mass of fly ash.