Abstract: The casing wear in deep and ultra deep wells is so serious that the increase of the extension limit of deep and ultra deep wells is restricted. In order to improve the extension limit of deep and ultra deep well, it is necessary to study the relationships between the casing wear and the extension limit of deep and ultra deep wells and definitely establish a deep-well extension limit model under the constraint of casing wear. In this paper, the energy equation of casing wear was established. Then, based on the geometric equation of casing wear section, the extension limit of deep and ultra deep well under the condition of casing wear was inverted by means of iteration method by introducing the wear depth limit of casing section and considering the load transfer of downhole string in the process of drilling. Finally, case calculation was carried out based on the data of a certain ultra deep well in Tarim. In addition, the rationality of the model was verified by comparing the calculation result with the caliper test data of the case well. According to the actual measurement result, the casing wear at the depth of 3 070 m in the case ultra deep well is the most serious with the wear loss of 3.15 mm. When this value is put into the proposed extension limit model under the constraint of casing wear, the calculated extension limit of the case well is 7 812 m, which is quite close to its actual well depth (7 500 m). It is verified that the proposed model is rational. The research results predict the extension limit of deep and ultra deep wells and provide theoretical basis for the safe operation and drilling tool type selection of deep and ultra deep wells from the perspective of casing wear theory.