The invention provides a three-dimensional positioning method for automatic slagging-off of desulfurized molten iron. The method comprises the following steps of establishing an original data model, calculating the minimum lifting height H1, the maximum lifting height H2, the minimum extension distance C2, the maximum extension distance C3 and the rotation angle value T of a slagging-off machine, and sequentially controlling the lifting height, the rotation angle and the extension length of the slagging-off machine according to the calculated data to carry out automatic slagging-off operation. The situation that errors are too large during manual operation and the slagging-off effect is sometimes good or sometimes bad is avoided, the influence of personal errors on the slagging-off effect is reduced, the inclination angle of a tank body during slagging-off is determined, three-dimensional coordinates needing to act of the slagging-off machine are calculated through a certain method, and the slagging-off operation is carried out according to the set coordinates; and meanwhile, the three-dimensional coordinates are continuously changed, an area needing slagging-off is determined, automation of the slagging-off process is achieved, manual participation is reduced, the slagging-off efficiency is improved, the workload of workers is reduced, and harm caused by molten iron spray explosion to operators in the slagging-off process is reduced.