Abstract: With the widespread application of line-commutated-converter based high voltage direct current (LCC-HVDC) technology, the harmonic interaction issue of AC-DC hybrid power grid is becoming more and more serious. The establishment of an LCC-HVDC small signal model is an essential approach to analyze the AC-DC harmonic coupling characteristics of the converter. The influence of harmonics on the LCC-HVDC system is analyzed by constructing a small-signal model of dual-terminal 12-pulse LCC-HVD based on the harmonic state space (HSS) theory. This model not only takes into account the LCC harmonic transfer characteristics and the connection method of converter transformers, but also considers the influence of time delay on the dual-terminal LCC system. The modeling process is simplified based on the idea of modularization, so that the harmonic state space modeling is easier to expand, thereby improving the accuracy of small-signal modeling for the dual-terminal LCC system. The specific expression of AC and DC harmonic transfer is provided and the accuracy of the proposed model is verified through simulation. The proposed model lays a solid foundation for both subsequent expansion or access to more complex system, as well as the interaction stability analysis and parameter design of the dual-terminal LCC system.