Steady-state probability, frequency and duration are the three main indices in reliability evaluation of the HVDC systems. Generally, they are solved by numerical methods. Their topolgical formulaes are derived by using the basic theory of ccates graph and considering conditions for some particularity of state-space in this paper. A set of generalised formulaes can be applied to obtain these indices expression about reliability parameters of components. Finally,an example is given to illustrate and verify this method.