Abstract: China Experimental Fast Reactor (CEFR) is of pool-type, and the pool is divided into two plenums which are named hot plenum and cold plenum. Triplex horizontal thermal-shielding board, situated between hot plenum and cold plenum, is a cold boundary of hot plenum, and a hot boundary of cold plenum too. Thermal-hydraulic characteristic of the triplex horizontal thermal-shielding affects the temperature distribution of hot and cold plenums directly. The thermal-hydraulic characteristics of the 1/4 domain of the triplex horizontal thermal-shielding were numerically simulated when CEFR was in full power by using CFX 3-D program. The boundary temperature of cold plenum is 360 ℃, while hot plenum is 490 ℃ or 516 ℃ of two projects. The selected model is RNG k-ε model. Compared with the results in existence, the gained average temperature is less than the designed temperature, for calculating convection heat transfer.