The invention relates to a high-voltage indoor line grounding alarming device which comprises an acousto-optic circuit, voltage relays YJ1 and YJ2, a time relay SJ and a relay ZJ. Two ends of the coil of the time relay SJ and the relay ZJ are connected with a positive end and a negative end of a control power supply bus. One end of a coil of the voltage relays YJ1 and YJ2 is connected with a zero line of a to-be-detected line, and the other end of the coil is grounded. Normally open contacts Y1 and Y2 of the voltage relays YJ1 and YJ2 are in parallel and then serially connected between the time relay SJ and the power supply bus. A normally open contact S1 of the time relay SJ is serially connected between the relay ZJ and the power supply bus. Normally open contacts Z1 and Z2 of the relay ZJ are serially connected between an alarm lamp and a loudspeaker in the acousto-optic circuit and the power supply bus. By the alarming device, 24-hour unmanned online detection of a 10kV line for grounding fault can be performed, human casualty accidents can be avoided, power failure maintenance after accidents can be reduced, and labor intensity of maintaining staff is lowered.