·Thirty resin-simulated curved single root canals were randomly divided into three groups. Root canal preparation was performed with a Blue file, an XP file or an M3 file randomly. Red or blue-black ink was injected into the root canal before and after preparation and photographs were taken under the stomatology surgical microscope. The images before and after preparation were overlapped. The apex of the prepared root was set as a circle center and the radius started at 1 mm. A concentric circle was made every 1 mm increased and sequentially incremented to 10 mm so that it intersected the image to form 10 measurement sites, and then the removal of ten measurement sites on the inside and outside of the root canal was recorded for statistical analysis. After the root canal was prepared the file was observed for tensile deformation under the stomatology surgical microscope.

·There was significant difference in the absolute value of the difference between the internal and external removal of root canal between the M3 group and the Blue group, as well as the M3 group and the XP group (P=0.003 1, P=0.002 0); but there was no significant difference between the Blue group and the XP group (P=0.350). The file body of XP group was observed to have de-helix deformation under the stomatology surgical microscope, which did not return to its original shape when placed in hot water, and no deformation was observed in the file body of M3 group and Blue group.

·The three kinds of single-file nickel-titanium instruments have good formation ability when preparing the curved root canals. But XP file and Blue file can maintain the original curvature of root canals better than M3 file. XP files are prone to deformation when preparing resin root canal, therefore the use of XP should be appropriately reduced clinically.