A power monitoring device for identifying the state of an electric appliance based on user feedbacks and a power monitoring method thereof are provided. The power monitoring device includes a measurement module, a variation detection and search module, and an electric appliance signature database established based on user feedbacks. When the measurement module detects a variation of a power consumption feature, the variation detection and search module searches the electric appliance signature database according to the variation to obtain electric appliances similar to the variation and the states thereof. A user contributes data to the electric appliance signature database by confirming the search result, or selecting the correct electric appliance and the state thereof. When the user confirms the search result, the power monitoring device establishes an association between the electric appliance and its electric appliance signatures and updates the electric appliance signature database.