Abstract: The stability of performance of testing completion fluid is key factor in high temperature and super deep gas well safety testing. During the completion well testing period of the vertical well section of Ku 1, we tested 2 times and the downhole packer was all imbedded, result in very serious downhole accident. According to the Ku 1 sidetrack drilling completion well formation testing and special request of well structure, through the laboratory study, testing completion fluid is the high performance combined completion fluid which adopts solid free formate completion fluid, formate drilling fluid viscous pad fluid, high density sulphonated polymer mud system, the system’s temperature resistance is above 168℃, and has powerful suspension property, and is free from pollution to the oil layer. The high density completion fluid through 25 days field tests,and after the completion fluid is static, the RD safe valve both in opens and remove plugging are all successfully. This solve effectively the wellbole stability of the complicated deep well under high temperature, high pressure and long hours testing condition, and testing stability under long hours static condition. This provides an example for the completion well safety test of other high temperature and high pressure well.