The core design of China Initiative Accelerator Driven System (CiADS) uses hexagonal wrapper tube fuel assembly with lead bismuth alloy as coolant. Due to the corrosive effect of lead-bismuth alloy, fuel assemblies are prone to blockage accidents, which lead to deterioration of heat transfer and endanger the integrity of the core. Therefore, the accident of the total instantaneous blockage (TIB) of a fuel assembly is one of design extension condition that CiADS needs to analyze. In this paper, TIB of a fuel assembly under normal operation of CiADS is simulated by CODA, a severe accident analysis code for liquid metal cooled fast reactor developed by China Institute of Atomic Energy. The flow heat transfer characteristics between the blocking assembly and adjacent assemblies are analyzed, and the safety boundary under TIB accident condition is obtained. The accident analysis results show that when the TIB accident occurred, the fuel assembly melted, the molten fuel rapidly diffused and melted the can wall of the adjacent assemblies, resulting in the loss of the integrity of the adjacent fuel assemblies, but the accident resulted in no more than seven boxes of fuel assembly melting. Based on the above results, the safety boundary of CiADS under TIB accident can be determined, and the accident acceptance criteria can be satisfied.