It is an important research content of operational reliability evaluation for the power system to calculate the failure probability of the relay protection system and quantify its impact on the outage of the primary component. A reliability model for the relay protection system is proposed to represent its principle， configuration and cooperation with other models. On the one hand， the so-called first-type failure is due to hardware failure， and its probability is calculated according to statistics. On the other， the hidden failure called the second-type failure will occur should the setting of the relay protection system not suit the power system operation. For this reason， the inactivity probability of the protection module is proposed to represent the effect of the power system fault type and protection principle， through which the second-type failure probability is calculated. Finally， the outage probability formulae of the primary component are derived with the relay protection system failure taken into account.