<p>Rapid cooling and solidification of polymer melts fundamentally limit jet stretching in melt electrospinning, particularly for polypropylene, which exhibits strong temperature dependent rheological behavior. In this study, the influence of ambient temperature on melt viscosity evolution, droplet formation, jet instability transition, and fiber morphology were systematically examined in polypropylene melt electrospinning. Within the processing range, the melt viscosity of polypropylene increased as the temperature decreased from 190 to 160 °C, suggesting a limited rheological window for stable jet deformation. Elevating the ambient temperature from room temperature to 120 °C effectively delayed viscosity growth, promoted transition from buckling dominated motion to whipping behavior between 80 °C and 90 °C, and enhanced electrohydrodynamic stretching. As a result, fiber diameter was significantly reduced, and more uniform fiber structures were obtained. Maintaining an ambient temperature of at least 90 °C stabilized the whipping regime and facilitated inter-fiber bonding. At 120 °C, regulation of ejection pressure further controlled melt throughput, reducing average fiber diameter from 9.66 ± 2.13 μm at 4 kPa to 2.50 ± 3.00 μm at 0 kPa, where the possibility of nanofiber formation was observed. However, excessively low pressure resulted in discontinuous spinning and increased diameter variability, demonstrating that an optimal balance between ambient temperature and ejection pressure is required. These results provide quantitative insight into how the ambient thermal environment primarily governs jet instability and fiber refinement, while ejection pressure modulates melt throughput within this thermally defined processing window.</p>