The strong nonlinearity of line commutated converter high voltage direct current (LCC-HVDC) leads to complexity and diversity of its internal frequency coupling, and the traditional modeling method finds it difficult to give consideration to accuracy and practicality. Therefore, a reduced dimension harmonic state space (HSS) modeling method based on the phase shift principle is proposed. The reduced dimension HSS model of an LCC-HVDC system is established by combining the harmonic domain phase shift principle with HSS theory. The time-domain simulation model of the LCC-HVDC system is also built in PSCAD/EMTDC to verify the correctness of the model. The small disturbance stability of an LCC-HVDC system is analyzed, and the dominant factors of the instability mode are identified by participation factor. Then, based on the established model, the influence of HSS truncation order on model accuracy and stability analysis is studied, and suggestions on the selection of HSS model truncation order of an LCC-HVDC system are given. The results show that the proposed model has higher integrity and accuracy, and the dimension is reduced by half compared with the traditional HSS model. This greatly shortens the calculation time and effectively reduces the complexity of theoretical analysis.