Abstract: To study the non-uniform stress and plastic deformation in threads of a bolted connection, a new method was proposed to detect three-dimensional weak magnetic signals under static tension loads within geomagnetic field. The triangular thread with a special hatch on the nut was used for static tension testing, and a two-dimensional magnetic memory detecting transducer was used to get weak magnetic signals of all the three dimensions on site. Then, the stress and strain of the thread was calculated using finite-element-method and the relationship between weak magnetic signals and stresses of the three dimensions was analyzed. The result shows that there exists a strong correlation between the magnetic signals and stress.