To improve the accuracy of operational risk evaluation， an algorithm of operational risk evaluation is proposed， in which the time-variation of component's operational state and the effect of under frequency load shedding are considered. In this algorithm the components are divided into two types， and the time-dependent outage probability of each type is deduced based on daily generation scheduling. Moreover， the impact of under frequency load shedding is taken into account. At last， the algorithm develops a series of risk indices， including loss of load expectation， off-limit indices and state indices. The calculation and analysis of IEEE-RTS 79 reliability test systems show that the proposed algorithm is effective.