The iron losses in the motor of motorized spindles have a significant effect on the heat generation, working efficiency, and speed-torque characteristics of motorized spindles as well as on their thermal deformation and machining accuracy. The existing finite element and analytical methods based on Maxwell’s equations are too complicated to be suitable for engineering designers. A fast engineering method for estimating iron losses in the spindle motor is presented based on equivalent magnetic circuit (EMC) where the problem of solving a complex electromagnetic field inside the spindle motor is simplified into a simple magnetic circuit calculation by the assumption that the magnetic flux density distribution of any cross section along the magnetic flux direction in the spindle motor is uniform. The EMC is combined with the Boglietti’s model. They are integrated into a developed program by compiling source codes to achieve the analysis and prediction of iron losses in the spindle motor. The results obtained from the proposed method are compared with the prototype experiment data to verify its validity. With the purpose of ensuring accurate experiment results of iron losses, a method of no load running combined with a sudden loss of power supply is proposed to eliminate the braking torque and electromagnetic losses of the spindle motor, namely to achieve the separation of the mechanical loss from the total losses.