Abstract: In single-phase grid-tied inverters (GTI), the closed-loop synchronization based on phase-locked loop (PLL) is widely used. However, the negative-resistance characteristic introduced by PLL at low-frequency band is not conducive to the GTI stability. When the grid impedance is large, the GTI is prone to serious instability. Hence, extensive studies on the optimization or substitution of PLL are ongiong at home and abroad, in which. open-loop synchronization (OLS) itself is completely stable due to its non-feedback structure, thus receiving much attention in recent years. However, OLS owns obviously nonlinear characteristics, and it is difficult for it to obtain an accurate linearized small-signal model. The stability of GTI with OLS under very weak grid is still unclear. Therefore, this study focuses on the small-signal modeling and stability of GTI with OLS. Inspired by harmonic linearization theory, the linearization modeling method of OLS is proposed, and the output impedance model of GTI with OLS is established. On this basis, the robustness of OLS-GTI is discussed. It is found that the GTI with OLS exhibits excellent stability even under conditions of large grid impedance. The simulation and experimental results verify the accuracy of the above model and the adaptability of OLS under very weak grid.