The invention discloses a voltage sag detection device. The voltage sag detection device comprises a data acquisition module used for acquiring an external voltage signal; an absolute value calculation module used for performing absolute value calculation on the voltage sent by the data acquisition module; a sliding data window used for receiving a current value from the absolute value calculationmodule and storing the current value according to the time sequence; a difference value storage window used for storing the difference of absolute values between the current period of each sampling moment and a moment corresponding to a previous period; a mean value calculation module used for calculating the mean value of the sliding data window after being triggered when the data of the slidingdata window is updated; and a detection and alarm module used for detecting the values of the mean value calculation module and the difference value storage window and sending out an alarm signal. According to the invention, the sliding data window is established. When the sliding data window slides forwards, the data of the absolute value calculation module can transmit data to the sliding window in sequence. Therefore, a reliable flow control mechanism is provided for data transmission, and the accuracy of voltage sag detection is improved.