The invention provides a forklift-based prefabricated flue plate mounting tool and a construction method thereof. The mounting tool comprises a forklift, a hoist boom and a hoist connecting part. Thehoist boom is arranged on the forklift. The hoist connecting part is connected with the hoist boom. A flue plate is hung below the hoist connecting part. The construction method comprises the steps that the indication sign of flue plate mounting work is mounted; the long side of the flue plate is placed at the designated spot in the tunnel direction; the flue plate is lifted by the mounting tool;the flue plate and the center axis of a tunnel are adjusted to form the posture of a 45-degree angle; the flue plate is lifted to be above concrete bracket columns, and the long side of the flue plateis adjusted to be perpendicular to the center axis of the tunnel; and the flue plate is lowered and adjusted to conform to the mark position of concrete brackets. The mounting tool is simple in structure and convenient to operate, mounting of the flue plate can be carried out in narrow tunnel space, and the subsequent procedure is not influenced.