Abstract: Coercive force testing is a nondestructive testing technique for detecting stress, which utilizes the relationship between coercive force and stress to accurately evaluate the stress state inside a material. Based on the theory of force magnetic coupling, the relationship between the coercive force and stress at different positions of 12Cr1MoVG specimens, 12Cr1MoVG welded specimens of the same type of steels, and 12Cr1MoVG/P91 welded specimens of different types of steels under different magnetization directions was studied. The results showed that the coercive force which direction of magnetization parallels to the direction of stress decreased first and then increased with the increase of stress. The coercive force which the direction of magnetization perpendicular to the direction of stress increased with the increase of stress. During the stretching process, the stress of the 12Cr1MoVG welded specimens of the same type of steels was concentrated in the base metal zone and heat affected zone. The stress of the 12Cr1MoVG/P91 welded specimens of different types of steels was concentrated in the 12Cr1MoVG base metal zone and its heat affected zone. The research results provided theoretical basis for the detection of coercive force in ferromagnetic materials.