Double-row different diameter ball slewing bearing has three seats. The steel balls and the spacers may be directly arranged into the upper and lower raceways. Two rows of steel balls with different diameters are fitted according to the load. Such open mode fitting features extraordinary convenience. The load angles of both upper and lower races are 90°, which enable the bearing to bear large axial force and the overturning moment. When the radial force is larger than 1/10 of the axial force, the raceways should be especially designed. The axial dimension and radial dimension of double-row ball slewing bearing are rather large, and the bearing construction is sturdy. Hence it is especially suitable for tower cranes which requiring working diameter over medium range, mobile cranes and other unloading machines.