A humidifier includes a base housing having a receiving cavity and an air outlet, a water tank having a vapor outlet, a vaporizing arrangement having a vaporizer and a fan, and a waterproof arrangement. The waterproof arrangement includes a blocking member movably mounted on the base housing to selectively move between an opened position and a closed position, wherein when the water tank is detached from the base housing, the blocking member is driven to block the air outlet so as to prevent water from passing through the air outlet, wherein when the water tank is attached on the base housing, the blocking member is driven to unblock the air outlet so as to allow air created by the fan to flow through the air outlet.