Due to its special structural form, the reducers have common corrosion defects such as corrosion thinning, cracks and so on after being strongly impacted by the medium inside the pipes. However, there are few studies on the detection of reducers with the help of ultrasonic guided waves at present. In order to explore the effect of reducers to ultrasonic guided waves detection, the propagation characteristics of ultrasonic guided waves in reducer were simulated by using ABQUES. In the simulation, L (0, 2) mode was used to detect the reducer by two set of experiments. The first set of simulation experiment, L (0, 2) mode was excited from the large end of the pipe to detect the pipe with axial crack, circumferential crack and no crack respectively. The second set of simulation experiment, L (0, 2) mode was excited from the small end of the pipe to detect the pipe with axial crack, circumferential crack and no crack. The ability to detect defect was assessed by the time domain waveform of reflected echo, and the dynamic stress distribution cloud map is used to visually explain the propagation characteristics of the guided waves passing through the reducer, and the difference of detection capability between the two ends of excitation is judged by comparing the detection results. The conclusion shows that it is more sensitive to detect defect when ultrasonic guided waves are excited at the larger end, the ability to detect defects is weak when ultrasonic guided waves are excited at the small end. The mode conversion occurs and generate F (1, 1) mode, when the guided waves pass through the reducer from both ends. This paper provides effective technical guidance when ultrasonic guided waves detect defect on reducers.