The electrical conductivity of trachyteandesite was measured in situ under conditions of pressure range from 0.5-2.0 GPa and temperature range from 773-1,323 K using a YJ-3000t multi-anvil press and a Solartron-1260 Impedance/Gain-phase Analyzer. The experimental results indicate that the electrical conductivity of trachyteandesite increases with increasing temperature and decreases with a rise in pressure. The relationship between the electrical conductivity (σ) and temperature (T) conforms to the Arrhenius equation within a certain temperature range. When the temperature rises to 923 K, the electrical conductivity of trachyandesite abruptly increases. This result demonstrates that trachyandesite begins to dehydrate at ~923 K and produces magnetite with a high-conductivity mineral phase after dehydration. The intergrowth of interconnected magnetite is the cause for the ~2 orders of magnitude increase in the electrical conductivity after dehydration. The interconnected high-conductivity mineral phase of magnetite in the dehydration product of the trachyandesite sample can be used to reasonably explain the high-conductivity anomalies in the South-Central Chilean subduction zone beneath the Andes.