The early fire monitoring of cable tunnels is very important to ensure their safe operation and maintenance. Partial discharge and cable overheating are the main causes of power cable tunnel fires. To discover fire hazards in time, experiments were designed for the partial discharge of epoxy resin and cables and for cable overheating. The experimental results showed that they will emit many particles with specific diameters and release volatile organic compounds (VOCs) during cable discharge and cable overheating; therefore, we propose that the count of emission particles with specific diameters and the concentration of VOCs can be used as criteria for discovering fire hazards in power cable tunnels. Finally, the implementation of the fire hazard alarm method is presented by monitoring the concentration trends of VOCs and particles. The research results can provide guidance for the design of power cable tunnel fire warning systems and related fire warning standards.