With the increase of converter-based renewable energy generation connected into the power grid, the interaction between renewable energy and grid impedance has introduced lots of new issues, among which the sub- and super-synchronous oscillation phenomenon makes a big concern. The linear active disturbance rejection control (LADRC) is a potential way to improve the damping characteristics of the grid-connected system, but the key factors and influencing mechanism on system stability are unknown. This paper establishes the equivalent impedance and coupling admittance models of a typical three-phase grid-connected converter. Then, the influence of the key factors such as the bandwidth of the LADRC and grid impedance on the stability and frequency coupling effect is assessed in detail. Finally, the theoretical analysis results are verified by simulations and experiments.