Considering the opening of stator slot, the precision of air gap flux density error correction directly affected accuracy of cogging torque. Correcting magnetic flux leakage of flux barrier, magnetic field intensity inside the rotor can be equivalent to the one on rotor surface. The mathematical models of opening slots and half opening slots were established respectively. The variation of magnetic path length was analyzed by partitions magnetic circuit, for illustrating mathematical correlation of cogging torque and different slot opening shape. The effect of various grades silicon steel sheet on cogging torque was analyzed. The study shows that the method above can access comparatively high approximation precision, the material determined by comparative analysis can reduce cogging torque effectively as well.