The invention discloses a substation direct-current (DC) bus alternating-current (AC) voltage serial-in monitoring device in order to solve the problem that the difference between modes of modification or replacement of DC bus monitoring equipment of different manufacturers leads to high cost of replacement and long time of replacement. The device comprises an AC voltage sampling module, a CPU control module and an alarm module which are connected in sequence. The AC voltage sampling module comprises a DC blocking circuit, a voltage sampling circuit, a voltage following circuit, a rectifier circuit, a filter circuit and an AD conversion circuit which are connected in sequence. The output of the AD conversion circuit is connected with the input of the CPU control module. The DC blocking circuit includes a DC blocking capacitor which is connected in series to the rear end of a DC bus to be detected. The voltage sampling module includes a voltage transformer which is connected in series to the rear end of the DC blocking circuit and is used for sampling the AC serial-in voltage in the DC bus to be detected. With the device, monitoring alarm can be raised for AC serial-in of a DC bus.