[Purposes] This paper aims to study the electrical properties of conductive asphalt mixtures and promote conductive asphalt pavements. [Methods] Multiphase composite conductive asphalt mixtures were prepared by using graphene, carbon fiber, and steel slag as conductive phases and subjected to electrical tests，so as to determine the optimal contents of carbon fiber and graphene and characterize the voltage-current characteristics and electrothermal characteristics of conductive asphalt mixtures under altermating current power. [Findings] For conductive asphalt, the optimal contents of conductive monofilament flocculent carbon fiber and graphene are 0.5% and 0.8%（mass fraction）, respectively. For conductive asphalt mixtures, the optimal content of conductive phase needle sheet carbon fiber is 0.35% （mass fraction）. Higher content of the conductive phase in the conductive asphalt mixture indicates a larger specimen size and higher linearity of its voltage-current characteristics. The electrothermal effect of conductive asphalt mixtures is mainly generated by carbon fibers. The temperature distribution of the specimen is not uniform after energization, and the temperature rise is fastest near the electrodes ends. [Conclusions] Multiphase composite conductive asphalt mixtures have good heating performance. The research results can provide some references for applying multiphase composite conductive asphalt mixtures in electrothermal snow-melting pavement.