The ultradeep carbonate reservoir in Sichuan Basin is characterized by deep burial depth, high temperature, and strong heterogeneity. In the early stage of production, the vertical well acid fracturing is the main reservoir stimulation method, and the horizontal well stimulation technology is not mature enough to release the production capacity of gas wells. Segmented acid fracturing of the ultradeep horizontal wells currently faces the following problems: the strong heterogeneity of reservoir leads to the difficulty of fine segmentation; the high reservoir temperature requires higher performance of working fluid; the reaction rate between acid and rock is fast and the action distance of acid is short, and there is low fracture conductivity under high closure stress. In view of the above problems, the fine segmented design method was studied, and the high-temperature-resistant authigenic acid and gelling acid systems were developed. The viscosity of authigenic acid is greater than 150 mPa s after shearing at 160 °C and 170 s-1 for 50 min, and the highest acid generation concentration is 4.05 mol/L. The gelling acid system has both the properties of high-temperature resistance and low friction resistance; not only canit meet the requirements of the retarding rate and the corrosion inhibition ability when the reservoir temperature is 160 °C but also the resistance reduction rate is up to more than 70%. By alternating injection of authigenic acid and gelling acid, the acid-etched fracture length and conductivity were, respectively, increased by 80% and 45%. The application of this technology in the horizontal well of the ultradeep carbonate reservoir in Sichuan Basin can increase the productivity by 3 times when compared with the vertical well acid fracturing, and a better stimulation effect has been achieved.