Abstract: Volume and pressure had coupled effect on each other, based on it the authors established mathematic model to calculate pressure in the sealed annulus by iterative method. With example in literature 1 to calculate pressure which changed due to temperature, the result indicated that the pressure in the sealed annulus based on this model was 10% less than that based on the absolutely rigidity casing model. The method could instruct well structure design and casing intensity design in HP/HT wells.