Space charge dynamics critically influence the insulation performance. While most existing studies have focused on uniform electric fields, a systematic experimental investigation into the influence mechanism of voltage polarity in non-uniform electric fields remains lacking. This work investigates space charge evolution in a non-uniform electric field under positive and negative DC voltages through a method combining experiments and simulations. The experiment was conducted based on the pulsed electro-acoustic (PEA) theory, and an epoxy resin sample with a needle electrode embedded was employed to create a typical non-uniform field. The simulation model reproduced the experimental observations successfully through multi-parameter iterative optimization. Results show a distinct polarity effect in charge behavior. Positive carriers possess a higher mobility, a higher ratio of trapping to detrapping coefficients, and a lower injection barrier than electrons. Thus, charge under positive voltage exhibits greater injection depth and density, but accumulates more densely near the needle tip, strongly suppressing the near-tip electric field and enhancing the distant field. Under negative voltage, the peak of the charge density exhibits a more distinct migration into the sample. These findings complement the insights into polarity-dependent degradation mechanisms.