The steering knuckle is a key component to connect automobile steering system and suspension system， and the performance of steering knuckles affects the incidence rate of road traffic accidents directly. In this paper， in view of the shortcomings and deficiencies of the defect detection methods and technologies of automobile steering knuckle， a new defect detection method of automobile steering knuckle based on Lorentz force is proposed. The new method generates Lorentz force in steering knuckle by controlling electromagnetic signal coupling. Explicitly， Lorentz force excites ultrasonic signals by vibrating solid particles in the steering knuckle， and then the ultrasonic signals are reconstructed to detect the defect distribution in the steering knuckle. In this paper， the basic principle of Lorentz force detection and the process of generating Lorentz force by controlling electric and magnetic signals are studied，and the plane model of the detection method is analyzed and simulated. The results show that the current density distribution， Lorentz force distribution and Lorentz force dispersion distribution in the steering knuckle can reflect the conductivity distribution of the steering knuckle in the coupled imaging detection method with Lorentz force being the sound source. The research work provides a foundation for application of the new method， which is beneficial to development of new detection technologies and applications of new detection methods in the automotive safety inspection industry.