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Polyamide 6 (PA6) materials with different degrees of long chain branching (LCB) were prepared via extrusion through varying the addition of chain extender. The LCB degree was quantitatively determined by using a rheological method based on macromolecular dynamic models. The topological structures of long-chain branching and crosslinked network were effectively distinguished through rheological percolation analysis so that modified PA6 with ultra-high LCB level was obtained. The XRD analysis indicated that LCB modification favored the generation of the γ-form, and the γ-form content of LCB-PA6 increased with increasing degree of LCB, which could provide a reasonable explanation for the high Izod impact strength of ultra-high LCB-PA6. Another reason for the great enhancement of impact toughness was the strong interfacial interaction between LCB molecules and virgin PA6 matrix. A large number of fibrillar ligaments and cavities during crack propagation were observed in the SEM analysis.