Abstract: It is very important for the design and safe operation of pool-type fast reactor to master the asymmetric three dimensional thermal hydraulic characteristics under the accident of one primary pump trip superimposing loss of off-site power. Due to the complex structure and multiple flow paths, it is difficult for the existing system codes to accurately obtain the three-dimensional thermal and hydraulic characteristics of the pool-type sodium-cooled fast reactor under the typical asymmetric accident. This study was based on the three-dimensional numerical calculation method, and a full-scale integrative CEFR cold hot sodium pool model was established. The three-dimensional transient thermal hydraulic behavior under asymmetric conditions of one primary pump trip superimposing loss of off-site power was simulated. In particular, the transient process of running down and reflux was simulated. Transient variation characteristics of three-dimensional asymmetric flow field, temperature distribution and the key thermal parameters, such as the IHX inlet and outlet temperature, were revealed. The calculation results showed that in the 1500 s, the coolant flowed upward back to the hot sodium pool through the IHX outlet of the trip loop and entered the cold sodium pool through the normal loop IHX outlet, and the average outlet temperature of the normal loop IHX appeared a maximum value of about 491.9℃ around 600 s, and then gradually decreased. However, the average outlet temperature of the trip loop IHX continued to rise and gradually tended to be consistent with the normal loop. The calculation results provided the key numerical reference for reactor safety evaluation and structural stress analysis under this working condition.