Different kinds of derivative accidents due to reaction runaway happen frequently. The research and the prediction of such accidents and their consequences are useful in the determination of safety distances and the establishment of emergency plan. The present paper takes into account the complex components and their dynamic change in the reactor, and proposes a new quantitative analysis method for the derivative accident consequences of reaction runaway. The so-called "simulated process for derivative accidents of reaction runaway" (SPDA-RR), based on the logic diagram of accident judgement, relies on Aspen Plus to describe the process simulation. The hazard analysis of such accidents involves many mathematical models for overpressure explosion, fireball, vapor cloud explosion, harmful gas dispersion, etc. The governing parameters of the mixture used in the mathematical models, such as the component content, temperature, pressure, enthalpy and entropy, can be obtained from SPDA-RR. Furthermore, in order to analyze other cases of reaction runaway, this SPDA-RR can be easily extended by an adequate parameter adjustment.