The utility model discloses a cross arm mounting and positioning device and relates to the field of power construction assisting tools. The cross arm mounting and positioning device comprises a first semicircular hoop, a second semicircular hoop and a connecting plate; one ends of the first semicircular hoop and the second semicircular hoop are connected through the connecting plate, and the other ends of the first semicircular hoop and the second semicircular hoop are provided with a locking device; the outside of the semicircular hoop is provided with a main plate, and the main plate is transversely provided with a cross arm fixing groove, which is formed at one end of the main plate, which is close to the second semicircular hoop; a leveling instrument and a compass are embedded into the upper surface of the main plate, a mounting case is arranged outside the main plate, and the inside of the mounting case is provided with a measuring tape. Compared with the prior art, the cross arm mounting and positioning device has the advantages of being capable of conveniently and rapidly mounting a cross arm onto a horizontal pole, ensuring mounting flatness of the cross arm, meeting the mounting standards of the cross arm, reducing the labor intensity of workers, saving time and labor and improving the working efficiency of the workers.