Optical tweezers for neutral atoms are a core platform for quantum simulation and precision metrology. In such experiments, tight laser foci are often generated with objectives of high numerical aperture (NA) through a vacuum window, where spherical aberration is inevitably introduced. This aberration in the wavefront stretches the focal light field, reducing the peak intensity and gradient, thereby degrading the trapping performance. Conventional solutions mostly rely on custom-made objectives or programmable wavefront-shaping devices to correct the aberrated optical field. However, to our knowledge, no quantitative experimental comparison has been made regarding the atomic transition frequencies and trapping lifetimes before and after the wavefront compensation. We demonstrate active compensation of the window-induced spherical aberration at the 813.4 nm magic wavelength of ^88Sr using a standard commercial microscope objective (20\times, NA = 0.4) combined with a phase-only spatial light modulator (SLM), with the phase mask analytically determined from the wavefront phase retardation of the 3.0-mm-thick fused-silica vacuum window. A 1:1 auxiliary optical path is constructed to quantitatively characterize the three-dimensional focal field before and after compensation. Trapping improvements are evaluated through the ^1S_0--^3P_1 transition frequency and the lifetime of the trapped atoms. Our results show that the axial and radial full widths at half maximum (FWHMs) of the spherically aberrated optical trap are significantly compressed with the correction phase implemented, and the effective intensity fraction is improved by 93.3% relative to the uncompensated case. At the atomic level, the ^1S_0--^3P_1 resonance frequency is further shifted to the red by 242.3 kHz after compensation, corresponding to a trap-depth increase of 48.1 \textµK. The 1/e trapping lifetime is extended from 799.9 ms to 2317.6 ms by a factor of 2.90. These results show that spherical aberration compensation significantly restores the focusing quality of the optical tweezer and increases the trap depth, suppressing atom escape losses and heating, thereby improving the trapping performance. This work provides a quantitative atomic-level evaluation of vacuum-window aberration compensation in optical tweezers, and offers a useful reference for related quantum information research.