Objective To explore the effect of ultrasonic irrigation technology on the removal effect of smear layer from root canal. Methods A total of 80 patients who received root canal therapy in our hospital from February 2024 to August 2025 were selected. According to the random number table method, they were divided into the control group and the experimental group, with 40 patients in each group. The control group was given conventional manual irrigation for root canal debridement, and the experimental group was treated with ultrasonic irrigation technology. The smear layer removal effect, root canal obturation quality, pain intensity and complication rate were compared between the two groups. Results The smear layer removal rate of the experimental group was 97.50%, which was higher than 82.50% of the control group (P<0.05). The qualiffed rate of root canal obturation in the experimental group was 95.00%, which was higher than 80.00% in the control group (P<0.05). The scores of VAS in the experimental group at 24 and 72 hours after operation were lower than those in the control group (P<0.05). There was no statistically signiffcant difference in VAS scores between the two groups at 1 week after operation (P>0.05). The incidence of complications in the experimental group was 2.50%, which was lower than 12.50% in the control group (P<0.05). Conclusion Ultrasonic irrigation is more effective than conventional manual irrigation in removing the smear layer from root canal. It can more thoroughly clean the root canal walls and reduce the impact of residual smear layer on treatment effect, thereby improving the quality of root canal obturation, reducing the early pain intensity and the risk of complications, which holds high clinical application value.