Abstract: When the existing deepwater oil and gas well completion technologies are brought into operation, the completion fluid is partially trapped in the casing annulus. And when the testing is carried out in deep water, the trapped fluid produces addition stress on the wellbore under the effect of hot fluid in high temperature and high pressure (HTHP) pay zones. In order to eliminate the damage on wellbore integrity, a trap pressure control technology based on the insulation tabular was designed for deepwater oil and gas well production testing. Based on the typical casing program of deepwater HTHP wells in South China Sea, a model for the deepwater wellbore heat conduction in the process of testing was established. After the wellbore heat conduction of typical wells was calculated, the predicted trap annulus temperature fields in the structures of conventional testing strings and insulated-tubing testing strings were numerically simulated and their trap pressure was calculated. It is shown that during the testing of deepwater HTHP oil and gas wells, the combined testing string technology based on insulation tabular can reduce effectively the addition stress effect of fluid in HTHP pay zones on the trapped casing annulus and avoid downhole accidents. This technology provides a new effective method for the safe and efficient testing of deepwater HTHP oil and gas wells.