The motor core stress produced during the manufacturing process and motor operation changes the electromagnetic characteristics of the iron core. Thus, research on electromagnetic calculations under the condition of stress is one of the key challenges to achieve more efficient designs of permanent magnet motors. In this paper, the stress equations of a rotor centrifuge and stator interference assembly are studied, and the changing regularity of stress and magnetic properties of electrical steel and its causes are analyzed theoretically. A device for measuring the stress and magnetic properties of electrical steel is designed and tested. Then, a model for iron loss calculations that includes stress is proposed for improved design. The effects of stress on loss, d-q axis inductance, and torque are simulated and compared using an IPMSM as an example. The feasibility and accuracy of this method are verified by comparing the simulation and prototype test results.