Abstract: In this paper, a transfer function model of linearized DC control loop is established based on the Laplace transform of the state space model for the line commutated converter based high voltage direct current（ LCC-HVDC） transmission system with constant DC current control strategy on the rectifier side and constant DC voltage control strategy on the inverter side. Gain margin, phase margin and maximum peak value of sensitivity function are used to quantitatively evaluate the influence of DC control loop bandwidth, inverter-side phase-locked loop band width and inverter-side AC grid strength on the small-signal stability of each DC control loop. The specific variation rules of stability margin of each DC control loop with different system parameters are revealed. The simulation results verify the correctness of the linear transfer function model and the theoretical analysis results.