Preventive maintenance (PM) to the components of an ultra high-voltage DC (UHVDC) transmission system may improve its reliability. The difficulty is how to determine the weak component needing PM, and how to derive the PM model showing the impact of a component’s PM parameters on system reliability. This paper derives a reliability sensitivity model of the UHVDC to determine the vulnerable component. The PM model for the weak component with its potential failure state is proposed. The state space is hierarchically aggregated based on the capacity to propose the reliability model of the UHVDC with the PM. The optimization model of UHVDC reliability to the PM parameter of vulnerable components is proposed. The numerical results verify the feasibility of the proposed PM and the effectiveness on improving the reliability of the UHVDC. This work is supported by the National Natural Science Foundation of China (No. 51877061).