Symmetrical phase-locked loop(PLL) can suppress frequency coupling due to its symmetrical structure, but the extra d-axis voltage route may decrease system’s phase margin in weak grid. To this end, this paper completely describes the interaction between d and q axes of symmetrical PLL-synchronized VSC system by the small-signal modeling. It further reveals the instability mechanism brought by its symmetrical structure, and demarcates the stability region of system in weak grid. Moreover, a symmetrical virtual damping strategy is proposed to enhance system’s stability, thus the adaptability of symmetrical PLL is greatly improved under weak grid conditions. The effectiveness of theoretical analysis is verified with simulation results.