The invention discloses a device used for detecting a direct current power supply connected to an alternating current power supply and direct current power supply ripples and used in a transformer substation. The device comprises a transformer substation direct current bus, a working power module (15), an alternating current voltage sampling circuit (3), a band-pass filtering circuit (4), a direct current voltage sampling circuit (8), a filter circuit (9), a ripple voltage sampling circuit (10) and the like. A CPU module (6) is connected with a voltage analog-digital conversion circuit (5), a direct current voltage analog-digital conversion circuit (10), a ripple voltage analog-digital conversion module (13), an alarm uploading communication interface (16), a display screen (7), a man-machine interaction keyboard (11) and an alarm output circuit (14) respectively. The device is used for quickly detecting whether the alternating current power supply is connected to the direct current power supply and giving an alarm in time when detecting an abnormity; the device also can be used for on-line monitoring of ripples of a direct current system charging unit, thereby being capable of greatly improving the reliability of the direct current power supply and safe operation of a power grid.