In order to realize comprehensive and meticulous reliability assessment of digital substation system with complex structure， firstly， the overall structural model of digital substation based on parallel redundancy protocol is constructed. Then a new reliability assessment method which is called goal oriented methodology is introduced to the field of reliability analysis of digital substation. The goal oriented model used for reliability assessment is established based on the structural model of digital substation， and the overall reliability parameters of the substation could be obtained by deducing the model. Finally， the influence of system success probability caused by the reliability variation of different components is analyzed with sensitivity analysis method.