In order to investigate the residual magnetic field (RMF) variations of initially undemagnetized ferromagnetic materials, tensile tests were conducted on the U75V steel specimens under tensile load with increasing amplitudes. It is found that the fluctuation of the unidimensional RMF reduces with the increase of load in the elastic stage, and remains relatively stable during the plastic stage. An effective way for characterizing deformation stages of the undemagnetized specimen is proposed by analyzing the RMF signals outside the longitudinal axis of the specimen. The two-dimensional vector distribution of the RMF indicates that the specimen tends to be magnetized as a rectangular magnet with an N pole and S pole at two ends with the increase of load. The possible reasons underlying the experimental results are discussed on the basis of the theory of the interaction between dislocation and domain.