Simultaneously improving the mechanical properties and processability of polyamide-6 (PA6) by chain extension in melting extrusion is usually very difficult. To address this issue, a novel small-molecule triepoxy chain extender (D-1) is designed, synthesized, and used to modify PA6. The rheological measurements show that the shear viscosity of PA6 containing 2 wt % D, decreases by 52.2% compared with that of the original PA6 at a shear rate of 1000 s(-1), which is in the shear rate range of thermoplastic processing in the industry. This interesting result is attributed to the generation of a long-chain-branched structure of the macromolecular chains of chain-extended PA6, which reduces the power-law index and aggravates the shear-thinning behavior of the PA6 melt. In addition, the corresponding notched impact strength increases from 62.4 to 105.9 J/m. As a result, this work provides a feasible way to prepare PA6 with excellent mechanical properties and processability in industrial production.