<p>Shaped charge jet plays an auxiliary role in window sidetracking of high‑strength thick‑wall casings. The wall thickness of the charge liner significantly affects the morphology, velocity, and casing penetration performance of the shaped charge jet. To further improve the performance of the trumpet‑shaped liner (TSL) in shaped charge jet‑assisted window sidetracking, a numerical simulation model for casing penetration using a trumpet‑shaped charge structure is established. First, the influence of different combinations of the curved section wall thickness <i>η</i>₁ and top section wall thickness <i>η</i>₂ of the TSL on the morphology and velocity of the metal jet in air is analyzed. Then, from the perspectives of inlet and outlet diameters, penetration area, and head velocity attenuation rate, the penetration performance of various <i>η</i>₁–<i>η</i>₂ combinations on the casing under different standoff distances is investigated, and the optimal wall thickness combination of the TSL is obtained. The results show that the influence of <i>η</i>₁ on the jet head velocity is greater than that of <i>η</i>₂. When designing the wall thickness of the TSL, <i>η</i>₁ should be prioritized and a relatively small value is recommended. The metal jet performance and casing penetration area achieved under the combination <i>η</i>₁ ≤ <i>η</i>₂ are superior to those under <i>η</i>₁ ≥ <i>η</i>₂. At standoff distances of 0.5D and 1D, the optimal wall thickness combination is <i>η</i>₁ = 1&#xa0;mm and <i>η</i>₂ = 2.5&#xa0;mm, indicating that the standoff distance has little effect on the optimal wall thickness combination.</p>