This paper analyzes the small-signal oscillation stability of a grid-connected VSG under weak gird connection. Firstly, simplified linearized model of grid-connected VSGs is established. Secondly, based on the Routh stability criterion theory, the small-signal stability criterion of grid-connected VSGs is derived. Further analysis of the stability criterion shows that the grid-connected VSGs can always maintain stable operation when the power grid strength or control parameter varies. Therefore, weak grid connection condition will not cause small-signal instability of grid-connected VSGs. Finally, simulation cases demonstrate and verify the correctness of the theoretical analysis.