In order to investigate operational risk effect of nuclear power plant, this paper analyzed its influencing factors, such as technology risk, environmental risk, management risk and human events, financial risk. A hierarchy structure model based on those factors was established, and compositing rule of evidential theory and AHP were adopted to evaluate the operational risk effect of nuclear power plant. A decision support system of operational risk effect of nuclear power plant (DSSORENPP) based on evidential theory was made. And the operational risk effect of Chernobyl nuclear power plant was simulated by this decision support system.