Abstract: [Abstract] Objective To evaluate the difference in cleaning degree of the root canal walls of the mandibular incisors between labial access opening and lingual access opening. Methods 18 pairs of extracted mandibular incisors were randomly divided into group A and B. In each pair of teeth, perform labial access opening and lingual access opening separately on the two teeth to prepare root canals, and use nickel-titanium instruments for group A, stainless steel instrument for group B. Then observe the cleaning effect of the root canal walls under the scanning electron microscopy（SEM）. Results ①The cleaning effect of root canal walls in all groups was the crown 1/3>middle 1/3> apical 1/3.②In the crown 1/3, there was no significant difference in the cleaning effect of root canal walls among the groups (P>0.05).③In the middle 1/3, group B with lingual access opening was the worst(P<0.05), and there was no significant difference among other groups(P>0.05).④In the apical 1/3, group A was superior to group B(P<0.05), and group B with lingual access opening was the worst(P<0.05). Conclusion The labial access opening contributes to improving the effect of root canal preparation of the mandibular incisors, combining nickel-titanium instruments to achieve the best effect