Abstract: In this study, DMSP satellite observations from 2015–2022 are used to systematically investigate the control of the interplanetary magnetic field (IMF) B_x over the strength and latitude distribution of subauroral polarization streams (SAPS), with particular emphasis on the SAPS response under different IMF B_x polarities in the two hemispheres and in the dusk and dawn sectors. The SAPS response to IMF B_x exhibits a marked hemispheric asymmetry. At dusk, SAPS are stronger in the Northern Hemisphere (NH) under IMF cone angles ≥ 135° (IMF B_x < 0). Conversely, in the Southern Hemisphere (SH), SAPS are stronger under IMF cone angles ≤ 45° (IMF B_x > 0). This hemispheric preference reverses on the dawnside. On the dawnside, NH SAPS is stronger when the IMF cone angle ≤ 45°, while SH SAPS is stronger when the IMF cone angle ≥ 135°. Further analysis indicates that SAPS enhancement is not caused by changes in the peak density of field-aligned currents (FACs), but is closely associated with alterations in their macroscopic morphology, particularly the latitudinal spacing between the Region 1 (R1) and Region 2 (R2) current systems. On the duskside, the widening of the FACs gap leads to an increase in integrated current, which drives a stronger polarization electric field and thus higher SAPS flow velocity. On the dawnside, the influence of IMF B_x on SAPS is mainly manifested as hemispheric asymmetries in SAPS intensity and magnetic latitude distribution. In addition, ion upflow in the duskside ionosphere exhibits a hemispherically asymmetric response consistent with that of SAPS: ion upflow is stronger in the SH under IMF cone angles ≤ 45°, and stronger in the NH under IMF cone angle ≥ 135°. This close correspondence with the SAPS response pattern indicates that SAPS plays an important role in driving vertical ion motion. In contrast, ion upflow on the dawnside shows a weaker response to IMF B_x and is not fully consistent with the SAPS variation, indicating that the physical processes associated with ion upflow differ between the dusk and dawn sectors. These results establish IMF B_x as an important driver of SAPS hemispheric asymmetry, providing critical insights for improving space weather forecasting models.