In order to analyze the influence of harmonics on the stability of line commutated converter based high voltage direct current (LCC-HVDC) system. It is urgent to establish an accurate model of LCC-HVDC considering harmonic coupling. Based on the theory of harmonic state space (HSS), an impedance model of 12-pulse LCC is established considering frequency coupling effect and control system. The constructed AC/DC harmonic impedance model can match the sweep frequency results in a wider frequency band. Finally, the correctness of the proposed LCC-HSS impedance model is verified by comparing the PSCAD electromagnetic transient simulation results with the HSS impedance model calculation results. The LCC-HSS impedance modeling method improves the accuracy of mathematical modeling for LCC converter stations and can adapt to impedance modeling of LCC converter stations in various modes, providing a more accurate model for stability analysis and parameter optimization of LCC systems.