Objective To explore the application effect of ultrasonic instruments in patients with large tooth defect treated by cast porcelain onlay restoration. Methods A total of 40 patients with large tooth defect who underwent cast porcelain onlay restoration in Yancheng Stomatological Hospital from September 2020 to September 2022 were selected as the research subjects, and they were randomly divided into control group (n =22) and experimental group (n =18). The control group was treated with conventional diamond bur for tooth preparation, and the experimental group underwent refinement of the inlay cavity and shoulder with ultrasonic instruments after conventional tooth preparation. The evaluation of the restoration margin, gingival index, apical microleakage, oral hygiene index and adverse reactions were compared between the two groups. Results The proportions of grade A in the shape of the marginal finish line and adjacent edge coloring in the experimental group were higher than those in the control group (P <0.05). There was no significant difference in the proportion of grade A in marginal adaptation between the two groups (P >0.05). The GI and apical microleakage in the experimental group were lower than those in the control group (P <0.05). The DI-S and CI-S in the experimental group were lower than those in the control group (P <0.05). The incidence of adverse reactions in the experimental group was 0, which was lower than 22.73% in the control group (P <0.05). Conclusion The application of ultrasonic instruments in patients with large tooth defect treated by cast porcelain onlay restoration can improve the marginal finish shape, enhance the satisfaction of adjacent edge coloring, improve the gingival index and oral hygiene index, reduce the degree of apical microleakage, promote oral health, and reduce the risk of adverse reactions.