Grid synchronization stability is an important research topic for grid-forming converters. To prevent instability and undesired oscillations, the small-signal modeling and stability analysis must be performed, which requires the line impedance information. Nevertheless, the line impedance may be unknown in most cases. As a consequence, the designed control parameters may fail to satisfy the stability requirement. To fill in this research gap, this article presents a geometrical approach to evaluate and improve the small-signal synchronization stability of grid-forming converters. The proposed analytical approach maps the line impedance variation on a two-dimensional plane and identifies the stability boundary in a geometrical way. As a benefit, the controller of grid-forming converters can be explicitly designed to achieve the robust synchronization stability. Finally, the feasibility of the proposed technique is verified by simulation and hardware experimental results.