Abstract: When the grid-forming converter oscillates interactively with the power grid, tuning the control parameters of the converter to improve its impedance characteristics is an important measure to enhance the interactive stability between the converter and the grid. Aiming to reveal the general rules, this paper analytically investigates the control parameter's influence on the impedance. Under non-extreme parameters, the active power loop parameters only affect the sequence impedance in a very narrow frequency band near the fundamental frequency; the voltage loop parameters have a significant impact on the sequence impedance in a wide frequency band, and the current loop proportional coefficient affects the sequence impedance in the middle-high frequency band. Based on the analysis results, the general parameter tuning rules are proposed, i.e., under non-extreme parameters, appropriately increasing the inertia or the proportional coefficient of the voltage loop, or reducing the integral coefficient of the voltage loop, are beneficial to avoid the interactive oscillation in the sub-synchronous frequency band. These conclusions provide a general guidance to the parameter tuning for the interaction stability improvement. Experimental results verify the correctness of the theoretical analysis.