The use of shaft parts in mechanical systems generates cyclic stress, altering stress levels in certain areas. This alteration leads to plastic deformation, ultimately affecting service life. This paper examines the tensile plastic deformation behavior of Q235 steel specimens processed by cold drawing and hot rolling using the second harmonic method of nonlinear ultrasonic technique. A nonlinear ultrasonic testing system was established. Specimens with varying plastic damage were tested, and time-domain signals of nonlinear acoustic waves were collected and analyzed. This analysis characterized the influence of different processing methods on material properties. The experimental results indicate that processing mode significantly influences the nonlinearity of the specimen. The nonlinear coefficient is higher in specimens after hot rolling compared to those after cold drawing. Specimens with varying degrees of plastic damage exhibit a larger second harmonic variation amplitude compared to those without plastic damage. The ultrasonic nonlinear coefficient of different specimens increases monotonically with the increase in plastic deformation. For cold-drawn specimens with 27% plastic deformation, the nonlinear coefficient increases by 45.8%. For hot-rolled specimens with 25% plastic deformation, the nonlinear coefficient increases by 35.23%. The experimental results provide guidance for studying the testing and evaluation process of tensile plastic damage in Q235 steel.