Ultrasonic characterization of non-metallic inclusions in steel can be largely categorized into two aspects, i.e., time-domain and frequency-domain characterizations. In general, characterization parameters in time domain are insensitive to the type, location, and size of non-metallic inclusions; meanwhile, traditional frequency-domain characterization parameters cannot represent the features of inclusions generally and accurately due to that they are calculated by several key points on spectrum curve. Therefore, critical spectrum parameters calculated by all points on spectrum curve are put forward to characterize the depth and size of non-metallic inclusion, including spectrum energy, waveform factor and kurtosis coefficient. Simulations and experiments of Al₂O₃ inclusions in galvanized sheet were carried out by 50 MHz immersion spot focusing ultrasonic transducer in pulse-echo mode to testify the validity of the proposed parameters. The depths of inclusions were 0.45–0.95 mm in simulation, while the sizes were 0.03–0.20 mm. Results of both methods indicate that the three new parameters all have specific relationships with the depth and size of non-metallic inclusions, and they are more sensitive to the change of the size. Furthermore, the qualitative characterization of the depth and size of non-metallic inclusion can be achieved by the new parameters. Especially, the quantitative characterization of inclusion depth is tentatively in reach.