Abstract: Weak grids are gaining considerable attention since power generation resources are remote from power loads, which results in low-frequency/harmonic oscillation. However, the conventional impedance-based approach pays more attention to the identification of line impedance stable operating points and the stability margin of the return-ratio matrix, but it cannot provide detailed system parameter stability region. Thus, this paper proposed a line impedance stability region analysis method for the weak grid to provide the guidance for practical system planning and related stabilization method researches. Firstly, the source impedance and load admittance matrices for the AC weak grid were separately built. Then, the forbidden region was constructed for the above interconnected system, and it was further transformed into the stability operation region. Compared with the norm-based impedance criteria, the conservatism of the proposed stability region is significantly reduced. Moreover, the solution process of the line impedance stability region was switched as the identification problem regarding Hurwitz matrix through mirror, translation and rotation mapping of the interconnected system return-ratio matrix, and the line impedance stability region was directly obtained by guardian map theory. Eventually, the simulation and experimental results verified the conservatism and effectiveness of the proposed method.