Abstract: The completion operation window, namely various extreme environmental conditions for implementation of completion operation, is of great significance for guiding the completion operation at deepwater subsea wellhead, and falls into running and scraping pipe window, production string running window and extreme connection window on the basis of completion operation modes. In the course of completion operation, the strength of riser, the strength of wellhead and conductor, the inclination angle of wellhead, accessibility of completion tool and the angle of flexible joint are the important factors which restrict the completion operation. By means of creating the coupling model of tensioner-waterproof string- wellhead and conductor and soil, the rotational angle of flexible joint at top and bottom of waterproof pipe which, as the most important limiting condition, affects the completion operation of deepwater subsea wellhead is researched and quantitated, and is used to form the determination method of operation window during the completion operation of subsea wellhead combining with other limiting conditions. Through the practical calculation for a prospecting well in the South Sea, this method definite the extreme environment conditions allowed for all modes of completion operation, which maybe the reference for the completion operation in the development of deepwater oil and gas field.