The tests of dielectric constant of asphalt mixture under different compaction degrees were carried out to study the relationship between the dielectric properties and density of the asphalt mixture. The specimens with different compaction degrees were prepared by gyratory compaction and the dielectric constants were tested on the high-temperature dielectric constant platform. The relative density of bulk volume of specimens was calculated based on the multiphase dielectric constant model of composite material, and then compared with the actual result measured by the water gravity method to verify the accuracy of calculation. The results show that the dielectric constants of mineral aggregates with different gradations and aggregates shall be different. The dielectric constant of mineral aggregate should be calculated based on the specimens with standard gradation and void ratio, without being limited to the traditional values taken from 7 to 9. The relationship between the dielectric constant and the relative density of bulk volume of asphalt mixture was established by the Rayleigh model. It is seen that the deviation between the calculated result and the result measured by the water gravity method is less than 1%. When the compaction is uneven at the same depth of the specimen prepared by gyratory compaction, the errors in the relative density of bulk volume and void ratio of specimen at the same depth could be 0.092 and 3.5%, and the relative density of bulk volume of specimen may decrease gradually with the thickness of compaction.