External root resorption may occur as a consequence of trauma, orthodontic treatment, bacterial infection or incomplete sealing of the root canal system, and lead to crater formation on the resorbed apex. This would deform the root apex surface, and cause loss of apical constriction. Depending on the extent of the resorptive process, different treatment regimens have been proposed. A 31-year-old male patient, right lateral incision gingival fistula, loose 2 degrees, imaging manifestations of external apical root absorption is blunderbuss, the radiograph of apical bone is very discrete, periodontal membrane space widened obviously. In the root canal machinery, chemical cleaning and disinfection of root canal, calcium hydroxide is conveyed to the apical region about 3 mm thick. After the root canal system was filled, the glass ionomer cements and composite resin was filled crownly. These 1-month、3-month、6-month、12-month radiograph showed that the repair of the root absorption area is good, the clinical manifestations of the tooth function is good, no loose. This case shows that calcium hydroxide is successfully used for external apical root resorption cases.