In this study, transient analysis code of SCWRs (TACOS), with the ability of simulating transients or accidents under both supercritical water (SCW) conditions and subcritical water conditions, has been developed with fortran 90 language, and simulation has been performed to the European SCWR fuel qualification test (FQT) system. The semi-implicit finite difference technique was adopted for the solution of coolant dynamic behavior in the loop. Furthermore, an illustration of numerical solution for the heat structure model and other models was presented. The code TACOS is then applied to simulate the Edward-O’Brian blow-down experiment to evaluate its capacity in simulating the fast blow-down progress. Therefore, the design basis accidents (DBAs) with the trans-critical transient were investigated for the SCWR-FQT system. The results by TACOS indicate that the SCWR-FQT with the existing safety system can be cooled effectively.