The invention discloses a device and method for monitoring electrical network voltage instant jittering. The device comprises a first relay, a bypass power supply unit and a monitoring unit, wherein the bypass power supply unit comprises an AC-DC rectification circuit, a super capacitance energy storage system, an inverter and a second relay; the monitoring unit comprises a voltage sensor and an MCU. The method comprises the following steps: when there is no electrical network voltage instant jittering, an electrical network supplies power to a load directly and simultaneously supplies supply to the super capacitance energy storage system; the voltage sensor acquires electrical network voltage signals real time and transmits the acquired voltage signals to the MCU; when the MCU determines that there is electrical network voltage instant jittering, the MCU controls the first relay to be cut off and simultaneously controls the second relay to be conducted, and at the moment the bypass power supply unit begins to supply power; and when determining that the electrical network voltage instant jittering tops, the MCU controls the first relay to be conducted and simultaneously controls the second relay to be cut off, and the electrical network supplies power to the load. The device and method provided by the invention, when being used for monitoring the electrical network voltage instant jittering, can keep load voltage constant and ensures the stability of an automation line.