[Analysis the effect on the stress of root canal wall of pulpless teeth by different degree of root canal preparation]. 
To analyze the stress variations of wall of root canal of pulpless teeth that resulted from different diameters of root canal.  On the basis of the finite-element model of the mandibular first molar that modified by routine, diameter of root canal was modified and enlarged so as to the diameter of root canal was 1/3 and 1/2 of root diameter, then modified models were loaded and the maximal stress of each part of teeth and wall of root canal was calculated.  The results revealed that enlargement of root canal diameter will bring on increase of stress of root canal wall at the orifice and coronal 1/3.  The stress difference of lateral loading was greater than that of vertical loading.  The principle of root canal preparation should be infected material removed and good shape of root canal gained.  Over-preparation is unnecessary and harmful.