Abstract: Test packers in China have unstable mechanical performance and high failure rate during operation, which thus fail to test ultra-deep, high-temperature, and high-pressure (HTHP) oil and gas wells. Therefore, in this paper, a hydraulic anchor and lower slip were adopted to realize bidirectional anchoring, and a J-shaped slot structure was employed to realize the mechanical repeatable setting, recycling, and other functions. The bypass hole was designed to balance the upper and lower pressure difference of the packer element during unsealing, so as to protect the element. As a result, an HTHP gas seal test packer was developed. The packer was designed with three elements, and FKM materials were optimized to improve the performance of the element. In the experiment, the API 19TT standard was adopted to simulate the whole process of complicated processes such as run-in-hole (RIH), shut-in pressure survey, and flowing pressure and acid fracturing measuring during well opening, so as to realize seven pressure reversals, with an experimental temperature of 204 °C and pressure of 105 MPa, as well as maximum absolute pressure of 140 MPa, reaching V1-TP gas seal grade. The packer was tested in one ultra-deep well. The setting position was 7300 m, and the one-time setting success rate was 100%. The successful development breaks the monopoly of formation test packer technologies in other countries, reduces test costs, and provides a reference for developing high-end tools in China.