This paper investigates the effect of grid impedance variation on harmonic resonant current controllers for grid-connected voltage source converters by means of impedance-based analysis. It reveals that the negative harmonic resistances tend to be derived from harmonic resonant controllers in the closed-loop output admittance of converter. Such negative resistances may interact with the grid impedance resulting in steady state error or unstable harmonic compensation. To deal with this problem, a design guideline for harmonic resonant controllers under a wide range of grid impedance is proposed, where the controller gain boundary region is derived based on the short circuit ratio. The nonlinear time domain simulations are presented to verify the theoretical analysis in the frequency domain, and the experimental results based on the analysis results are included.