CSR1000 (China supercritical water reactor, 1000MWe) was developed by NPIC, as BWR, the pressure vessel’s reactor and directly circulate loop was adopted, however, the coolant will encounter double flow pass in the reactor. The passive engineering safety feature was adopted and the reactor residual heat will be removed by natural circulation of coolant. As above character, loss of feed water or loss of offsite power will cause completely loss of forced flow accident, the flow in first pass of core will encounter flow inversion during this course, these factor make the LOFA(Loss Of forced Flow Accident) become one of the most limiting accident in CSR1000. The LOFA was analyzed by APROS. The result shows, during the short time of LOFA, the passive operation of HFT will mitigate the accident, and during long term, the passive residual heat removal system will function and maintain the core within the safety state.