Abstract: The manufacturing process-caused ovality of the oil casing significantly influences the extrusion strength of its own. However, for a long time this issue had drawn little attention from people concerned, and thus had not been tackled. To solve the problem, a mechanical model is set up according to the elastic-plastic theory, and quantitive FE analysis is conducted to determine the influence by casing ovality on its extrusion strength. It is concluded that casing ovality significantly influences its extrusion strength. Therefore the only way to increase extrusion strength of the casing is trying to reduce its ovality as possible as it can be.