An automatic control device and an automatic control method of display brightness are provided. Human fatigue index and environmental factors for dry eyes are combined as an indicator to dynamically adjust the display brightness. Heart rate variability (HRV) is used as the human fatigue index, and the environmental CO2 concentration is used as the environmental factors for dry eyes here.提供一種顯示器亮度的自動控制裝置及其方法。在此結合人體疲勞指數與眼睛乾澀之環境因數做為指標依據，藉以動態調整顯示器之亮度。上述之人體疲勞指數採用由分析心跳變異率所得之指標，而眼睛乾澀之環境因數採用環境二氧化碳(CO2)濃度為指標。
