An active fixed block track circuit comprises at least a fixed block section, comprising at least two rails corresponding to each other, each having an isolation connector at two ends thereof and connected to a power supply unit and a traffic signal; a short driving unit, connected to each of the two rails within the fixed block section; and a detection unit, connected to the short driving unit. In this manner, not only the previous rail traffic signal circuit function where no more than one train may be allowed into the same fixed block section is maintained, but also another train approaching from the rear may be prevented from entering the fixed block section when the detection unit at the fixed block section detects an exceptional case happening at a neighboring fixed block section so that the traffic signal unit is switched into a stop traffic signal.