Abstract: Extended reach well directional drilling technology has been widely applied for the efficient development of oil and gas resources in complex areas such as oceans, beaches, lakes and mountains. However, there are many challenges in extended-reach drilling, and there are multiple constraints, which include huge technical difficulties and high risks, which make the prediction and control of wellbore extension limit of great importance for safe drilling. On the basis of summarizing research progress on the extension limit of extended reach wells at home and abroad, the prediction model and distribution law of the extension limit were introduced. Also, a extension limit-based extended reach well optimal drilling design was proposed, and such extension limit theory was used to analyze the rotary drilling limits and completion string entry limits in the horizontal section of the Z–42 extended reach well in Sakhalin area. It was considered that the hoisting performance of drilling rig could greatly improve the wellbore extension limit, and the installation of rotary joint and drag reducer joints could increase the entry limit of the completion string. The research results showed that those prediction theories and control methods could provide scientific references and best practices for risk prediction, optimized design and safety control of extended reach well drilling.