Abstract: The interaction between the long transmission cable（LTC） and the grid-connected inverter can easily cause the risk of harmonic instability of the system.Ignoring the frequency-dependent characteristics of the line may lead to stability misjudgment.In order to evaluate the stability of the grid-connected inverter system,the distribution parameter characteristics and frequencydependent characteristics of LTCs are taken into account in this paper.Firstly,the equivalence of the AC system is carried out based on the vector fitting method,and the high-frequency state-space model of the grid-connected inverter system in the dq coordinate system is established with consideration of the inner and outer loop control,phase-locked loop and control delay.Secondly,the key influencing links leading to the harmonic instability are identified by using the participation factor analysis method.Meanwhile,the influences of the frequency-dependent characteristics of LTCs,delay time,controller parameters,cable length,and power grid impedance on the system harmonic stability are analyzed by using the root locus method.Thirdly,a method for improving the system stability is proposed based on the parametric sensitivity analysis and the matrix similarity transformation,and the harmonic stability of the improved system is analyzed.Finally,the verification is carried out on the RT-LAB based hardware-in-loop experimental platform,and the experimental results verify the correctness of theoretical analysis.