Abstract: A small signal analytical model of UHVDC hierarchical connection mode is established in this paper, where the model emphasizes the interaction between the converters through AC system and DC system respectively. The effects of control modules such as PI controller and phase-locked loop (PLL) can be taken into consideration. The eigenvalue analysis indicates how the changes of AC system strength and parameters in HVDC control system influence the stability of UHVDC hierarchical connection system. Finally based on the practical HVDC transmission project, a simulation case is built in PSCAD/EMTDC to demonstrate the sensitivity of stability with respect to system parameters. The results can facilitate the relevant decision on the proper control parameters and the quantitative investigation on the stability of UHVDC hierarchical connection mode system.