To solve the measuring problem of the hanger's position and attitude, an approach based on improved April Tag technology was proposed. The method adopted a special magnetic label fixing mode to reduce the difficulty and requirements of label installation. The position and attitude of the hanger were measured based on the spatial coordinate transformation theory, with the visual measuring results of the April Tag labels on the hanger and the key dimensions of the hanger. Meanwhile, the visual measuring results of the April Tag were screened based on the relative relationship between labels to enhance the accuracy. A positing and attitude measurement experiment was conducted on the hanger of the overhead transmission lines' tower assembled by floor-standing double boom derrick. The results show that the proposed approach can effectively and accurately measuring the spatial position and attitude of the hanger.