A study was undertaken to investigate the structure, electrical conductivity and dielectric properties of low-rank coals during microwave pyrolysis. The mechanism of dielectric response during microwave pyrolysis was also discussed. The pyrolysis conditions were a microwave power of 800 W and different radiation times; dielectric properties were measured using a vector network analyzer at a frequency of 2450 MHz, and the electrical conductivity was measured using a four-probe resistivity tester. Results show that with increasing microwave radiation time, the functional groups gradually cracked and fell off, and the structural order was gradually enhanced. In addition, the order of the structure increased the number of sp(2)-hybridized carbon atoms on a single plane and increased the delocalized electrons between the graphite crystal planes. The complex relative permittivity and resistivity of the semi-cokes exhibited a significant dependence on microwave radiation time and pyrolysis temperature, especially after 15 min of microwave radiation. By comparing the calculated and measured values of imaginary parts, it could be deduced that the electric conductive loss is also an important part of the microwave attenuation mechanism in the process of microwave pyrolysis, as important as the relaxation loss and the interface polarization loss.