The method of enveloping is used commonly in the instrument survivability evaluation in severe accident in nuclear power plants(NPPs). Due to the limitation of this method, this paper studies the method of circuit simulation by PSpice as complementary. Based on the databases of phenomenon and effects on instruments in severe accident, we analyze and summarize the key condition factors that threats most. And according to the characteristics of instruments in NPPs, we create the circuit simulation models that present the instruments in NPPs. Comparing the simulation results with the survivability of instruments in TMI accident, the result shows that the model is reasonable.