The utility model relates to an alternating current intrusion monitoring device for a low-voltage direct current system of a transformer substation. the direct-current anode input end of the direct-current air switch is connected with the direct-current bus anode of a transformer substation low-voltage direct-current system direct-current distribution panel; the DC cathode input end of the DC airswitch is connected with the DC bus cathode of the substation low-voltage DC system DC distribution panel; the direct current anode output end of the direct current air switch is connected with the direct current anode monitoring interface of the alternating current serial-in direct current monitoring device; the DC cathode output end of the DC air switch is connected with the DC cathode monitoring interface of the AC serial-in DC monitoring device; one end of an alternating current serial-in state switch of the alternating current serial-in direct current monitoring device is connected with asignal common end of the background monitoring alarm, and the other end of the alternating current serial-in state switch of the alternating current serial-in direct current monitoring device is connected with an alarm signal communication end of the background monitoring alarm. The device can alarm and monitor the alternating current intrusion situation of the low-voltage direct current system of the transformer substation.