The invention discloses an automatic heating and moisture expelling alarming device for a substation terminal box and a mechanism box, which comprises an AC incoming wire power supply, a heater and a control circuit, wherein the AC incoming wire power supply is connected with the heater via the control circuit. The automatic heating and moisture expelling alarming device for a substation terminal box and a mechanism box is characterized in that the control circuit comprises an indication lamp, an alarming horn, a resistor, a humidity sensor, an adjustable resistor, a triode, a diode, a zener diode, a thyristor, a time relay, an optocoupler and a current transformer. According to the automatic heating and moisture expelling alarming device for a substation terminal box and a mechanism box, the design is reasonable, the structure is simple, and manufacturing is easy; the heating and moisture expelling device inside the box can be automatically opened when certain humidity exists in the box and automatically closed when the box is dry, whether the heater works or not can be automatically checked, and when the heater fails or is damaged, the indication lamp and the alarming horn give a sound-light alarm; and thus the working efficiency of an operation and maintenance person is improved, and dangers such as scald and electric shock caused by traditional checking methods such as hand feeling and measurement by a multi-meter can be avoided.