Abstract: TACOS（Transient Analysis Code for SCWRs） code has been developed to analyze the transient characteristics for SCWR system.In the present study,the transient analysis of various categories events for SCWR with mixed spectrum core（SCWR-M） are performed using the TACOS code,and the comparative study of three different SCWR concepts is conducted to compare the transient thermal-hydraulic characteristics and safety performance.The total loss of flow accident,one pump seizure,reactivity reduced accident,turbine trip without bypass are calculated for SCWR-M.These transients cover the three main categories events of SCWR,which are flow rate abnormality,reactivity abnormality and pressure abnormality.The comparative study of the three concepts with different flow path designs indicated that the design of multi-pass core could have more complicated transient flow mode,and would face greater challenge for the mitigation of total loss of flow accident.