A flank milling tool positioning method using a barrel cutter is proposed. An offset point is used as the first anchor point. Two rotary angles of the barrel cutter are adjusted to find the optimized tool position with the largest machining strip width. The result tool position calculated using the proposed method is gouge-free because the local interference avoidance method is integrated inside the tool positioning procedure. Error distribution beneath the barrel cutter is well estimated by virtual of the instant envelope curve of the cutter. The envelope curve is discretized into points. The distances between these points and the model surface are the machining errors beneath the cutter. The employment of the envelope curve also largely reduces the computational load of the algorithm. Finally, numerical implementation and simulation are performed to validate the feasibility of the method.