Objective To compare the efficacy of resin inlay and prefabricated metal crown in the restoration of large defect of deciduous molar in children after root canal treatment. Methods A total of 40 children with large defect of deciduous molar diagnosed and treated in our hospital from January 2020 to January 2024 were selected as the research subjects. They were divided into the control group and the research group by the random number table method, with 20 children in each group. The control group was treated with prefabricated metal crown restoration, and the research group was treated with resin inlay restoration. The clinical efficacy, masticatory function, periodontal health indicators (gingival index, plaque index) and complication rate were compared between the two groups. Results The restoration success rate of the research group was 95.00%, compared with 90.00% of the control group, the difference was not statistically significant (P >0.05). The maximum occlusal contact area, occlusal force and masticatory efficiency of the two groups after treatment were greater than those before treatment, and those of the research group were greater than those of the control group (P <0.05). The gingival index and plaque index of the research group were smaller than those of the control group (P <0.05). The incidence of complications in the research group was 5.00%, which was lower than 15.00% in the control group (P <0.05). Conclusion The prefabricated metal crown restoration is more effective in in the restoration of large defect of deciduous molar in children after root canal treatment. It can improve masticatory function, improve periodontal health, and reduce the incidence of complications, which is worthy of clinical application.