To quantify reliability and parametric sensitivity of the ultra HVDC (UHVDC) system, hierarchical models for reliability equivalent and improved F&D method are newly proposed. Based on the system configuration and the operation characteristics, multiple capacity levels, transformer winding connections, and single station sharing spare are considered. The UHVDC system is classified to several sub-systems, whose state spaces are derived and combined to yield the whole state space. The traditional frequency and duration (F&D) method necessitates manual search and calculating transition frequency to acquire transition rate, which is fallible, and the calculation efforts increases with the states. Improved F&D method based on matrix description yields relation of the transition matrices before and after equivalence. Sensitivity between system reliability indices and components’ reliability parameters is finally derived. Numerical results show that the proposed model and algorithm can quantify the reliability of UHVDC system and determine the critical parameter and weak component to unavailability of UHVDC system, which validates feasibility and effectiveness of the proposed model and algorithm.