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Objective To compare the efficacy of four methods on removal of calcium hydroxide in root canals. Methods Forty simulated resin root canals with 45° curvature were randomly divided into four groups. After root canal preparation, the apical foramen were sealed with glass-ionomer cement, the root canals were filled with calcium hydroxide, and the root canal orifices were sealed with hard wax. After being placed in a constant temperature water bath at 37 ℃ for 14 days, four methods including needle irrigation, Er: YAG laser irrigation, EDDY working tip irrigation and ultrasonic irrigation were used to remove the calcium hydroxide. The resin root canals before and after calcium hydroxide removal were scanned by cone-beam CT, and the scanning results were imported into Mimics software. Three-dimensional reconstruction of each root canal was performed by Mimics software, and the volume of calcium hydroxide in each root canal was calculated. The volume of calcium hydroxide was V1 before removal and V2 after removal, and the clearance rate was (V1 -V2)/V1 * 100%. Rank sum test was used for statistical analysis. Results There was no significant difference in the initial volume of calcium hydroxide paste among the groups before removal (P > 0.05). After removal, the residual volume of calcium hydroxide paste in Er: YAG laser group was 0.129 6 (0.092 0, 0.174 3) mm³, which was less than the other three groups, and the difference was statistically significant (P < 0.05). The reduction rate of calcium hydroxide paste was 99.63 % (99.53%, 99.70%) in Er: YAG laser group, which was higher than the other three groups, and the difference was statistically significant (P < 0.05). The reduction rates of EDDY group and ultrasonic group were 99.08% (98.85%, 99.19%) and 99.04% (98.51%, 99.15%), respectively. There was no significant difference between the two groups (P > 0.05), but they were higher than needle group 74.99% (64.43%, 88.07%) (P < 0.05), and the difference was statistically significant. Conclusions In this study, Er: YAG laser-activated irrigation, EDDY irrigation and ultrasonic irrigation have better removal effects on calcium hydroxide paste in root canals than traditional needle irrigation. Compared with other four methods, Er: YAG laser-activated irrigation has the highest removal efficiency, EDDY irrigation and ultrasonic irrigation have similar removal efficiency, but none of the four methods could completely remove calcium hydroxide paste from the curved root canals. [ABSTRACT FROM AUTHOR]