The present invention provides an alternating current CT energy taking device and an overvoltage prevention circuit. The alternating current CT energy taking device comprises a CT energy taking coil, a first rectification module, an energy storage capacitor and an output module; the CT energy taking coil is connected with the alternating current side of the first rectification module, the direct current side of the first rectification module is connected in parallel with the energy storage capacitor, the energy storage capacitor is connected in parallel with the output module, the output module comprises an output interface configured to connect with a load, a high-voltage monitoring module connected in parallel with the energy storage capacitor and a bypass module connected in parallel with the energy storage capacitor, the high-voltage monitoring module is configured to monitor the voltage of the energy storage capacitor, when the voltage of the energy storage capacitor is larger than a setting voltage value, the investment of the bypass module is controlled, and the output is performed through the above bypass module. The alternating current CT energy taking device selects an appropriate CT current for charging the energy storage capacitor to allow the work voltage of the energy storage capacitor into a certain range so as to ensure the voltage stabilization performance of the alternating current CT energy taking device and avoid high output power to cause load damaging when the CT current is large.