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Purpose: To evaluate the safety and effectiveness of passive ultrasonic irrigation(PUI) in curved root canals, and debris cleaning ability and the amount of root canal transportation of passive ultrasonic irrigation(PUI) in curved root canals., Methods: A total of 36 mesially curved root canals of mandibular molars with a curvature above 25° were selected. The root canals were prepared with XP-endo Shaper root canal file. The samples were divided into A1 group (curved length＞3 mm, syringe irrigation), B1 group(curved length＞3 mm, PUI+K file), C1 group(curved length＞3 mm, PUI+irrisafe), A2 group (curved length＜3 mm, syringe irrigation), B2 group (curved length＜3 mm, PUI+K file), and C2 group (curved length＜3 mm, PUI+ irrisafe) (n=6). Micro-CT scans were performed on all samples before and after irrigation, and the volume increase in the root canal after irrigation and the transportation of the root canal were calculated. SPSS 22.0 software package was used to analyze the data., Results: In the apical area of the root canal with a curved length greater than 3 mm, the root canal volume increase in the PUI+irrisafe group was significantly greater than that of the PUI+K file and syringe irrigation (P＜0.05), and at 5 mm section, the transportation of the root canal formed by PUI+irrisafe was significantly lower than that of the PUI+K file (P＜0.05), but there was no significant difference from syringe irrigation (P＞0.05); in root canals with a curved length less than 3 mm, root canal volume increment of the file group was significantly greater than that of syringe irrigation (P＜0.05), but there was no significant difference between the root canal transportation and syringe irrigation(P＜0.05)., Conclusions: In root canals with larger curved length, passive ultrasonic irrigation combined with a pre-curved file can obtain a better cleaning effect, while in root canals with smaller curved length, both K file and pre-curved file with passive ultrasonic irrigation are safe and effective.