The embodiment of the invention discloses an automatic dimming emergency lamp. The automatic dimming emergency lamp comprises an emergency power supply, a boost constant current module, a power supply switching module and a field effect transistor Q2, a load of the boost constant current module comprises a first load resistor and a second load resistor, two ends of the first load resistor are connected with ground and one end of the second load resistor respectively, the other end of the second load resistor is connected with a source electrode of the field effect transistor Q2, a drain electrode of the field effect transistor Q2 is grounded, a grid electrode of the field effect transistor Q2 is connected with a first input end of the power supply switching module, the first input end of the power supply switching module is connected with a mains supply conversion low power supply, a second input end of the power supply switching module is connected with the emergency power supply, an output end of the power supply switching module is connected with the boost constant current module, the power supply switching module is used for controlling the boost constant current module to access to the mains supply conversion low power supply when a mains supply is normal, the field effect transistor Q2 is connected, when the mains supply is cut off, the power supply is switched to the emergency power supply, and the field effect transistor is disconnected. According to the automatic dimming emergency lamp, the access or cutting off of the second load resistor is controlled through connecting and disconnecting of the Q2, so that the illumination intensity of the emergency lamp is changed, and the illumination time is prolonged.