Ultra-deep slim-hole downhole operations are confronted with significant challenges due to the complex wellbore environment, the high likelihood of fish falling, and the limited operational window, which severely restrict the safe and efficient execution of workover operations. To address these issues, Well TW1 on the southern margin of Xinjiang was selected as a representative case. By analyzing the well conditions and the fish-dropping process, the primary causes of fish loss were identified, and the key technical difficulties associated with slim-hole fishing were clarified. Fishing tools and operational procedures suitable for slim-hole constraints were optimized, and the fishing performance was evaluated with corresponding improvement suggestions proposed. Furthermore, based on the milling operation in Well TW1, a string mechanics analysis program was established using field-measured data. The friction coefficients of the cased hole and open hole were back-calculated, and a comprehensive mechanical strength evaluation, torque-transmission modeling, and sensitivity analysis were conducted. The results indicate that all hydraulic fishing tools with internal chambers are susceptible to deformation failure, and accurate identification of fish status is essential for fishing success. The friction coefficients are determined to be 0.15 in the cased section and 0.55 in the open-hole section. The well interval from 6 509.73 m to 8 012 m is prone to buckling. Axial force decreases markedly with increasing weight on bit and slightly with increasing mud density. Although surface torque increases with depth, the effective torque at the bottom is only 0.12 kN·m, showing distinct stepwise attenuation. It is therefore recommended that weight on bit and mud density be moderately reduced to improve bottomhole torque transmission. The research results provide theoretical guidance and engineering references for optimizing fishing technology and ensuring mechanical safety under complex ultra-deep slim-hole conditions.