AbstractLightweighting of automotives is crucial for the decrease of fuel and energy consumption, as well as CO2 emission and environmental pollution. Polymer composites, especially the high strength glass fiber/polyamide (GF/PA) composites, have great potential in lightweight; however, they have not been universally accepted due to their insufficient performance. Here, a stearate‐ended hyperbranched polyester (HBP‐12‐n) is designed and high‐performance GF/PA6 composites with high GF content are prepared. The flowability of the composites can be further increased by combining carboxyl‐ended hyperbranched polymer (HyPer C181) with HBP‐12‐n. Compared with unmodified GF/PA6 composites, the melt flow index (MFI) can be increased remarkably to 587% by adding a 0.7 wt% mixture of HBP‐12‐75 and HyPer C181, and the strength and toughness remain competitive. The simultaneous improvement in MFI and impact strength of GF/PA6 composites is the first discovery of this kind, and a synergistic mechanism of amphiphilic bridging‐deformation slipping‐interfacial strengthening of hyperbranched polymers is proposed to account for this phenomenon. This work provides a simple strategy to fabricate GF/PA6 composites with the extensive application for automotive parts and has great prospects in making automotives lightweight and reduction of CO2 emissions.