[en] The self-designed integrated small module reactor (SMR) operates with boric core, which determines that the boron transportation and reactivity feedback process cannot be simulated by system code THEMIS. In this study, a simulation method for boron reactivity was established based on the best estimation system code RELAP5, then the sequence of a postulated main steam pipeline broken accident of domestic generation III nuclear power plant was analyzed basing on this method to validate this method. At last, the postulated boron dilution accident sequence of self-designed SMR was simulated; the result showed that the core will return to be critical at 5110.0 second after the starting of the accident, and corresponding countermeasures should be taken to cease the boron dilution process. (authors)