An auto-darkening filter of a welding shield has a brightness-sensing unit, an optical shutter and a processor. The brightness-sensing unit generates a first signal or a second signal. In a high-sensitivity mode, the processor controls the optical shutter to operate in a dark state or a bright state depending on intensity of the first signal being higher or lower than a threshold value. In a low-sensitivity mode, the processor controls the optical shutter to operate in the bright state when intensity of the second signal is lower than the threshold value, and automatically change to the high-sensitivity mode from the low-sensitivity mode when intensity of the second signal is higher than the threshold value.