Abstract: Flow instability of supercritical water-cooled reactor CSR1000 was studied based on frequency domain. Mathematics model of core in CSR1000 was established. A code named FREDO-CSR1000 was developed to analyse the flow instability of CSR1000 both in average channel and hot channel within rated power and flow. Some key parameters including decay ratio, the proper orifice pressure drop coefficient and the stable operation region were also obtained. The calculation results show that the decay ratio of the first flow path varies with power and flow monotonously, and the decay ratio of the second flow path ascends and descends sharply because of the influence of inlet temperature.