Abstract: Gas and liquid concurrent downward flow through sieve plate packing is an effective contact mode. Industrial equipment involving this flow mode has been widely recognized due to its advantages of simple structure, high operating flexibility, no liquid flooding, and so on. Typical application of such flow mode is the degassing in radioactive wastewater treatment boron recovery system (TEP), in which the fission gas accounts for more than 90% of the radioactive components. In order to significantly reduce the ratio of purification, and also to ensure the heat transfer of the evaporative sequence condenser in subsequent sections, it is required that the degassing rate of the TEP system must be greater than 99%. The crucial equipment of degassing in TEP system is the sieve plate packing tower, in which the sieve orifice is the basic unit. Through orifice contraction and expansion to enhance gasliquid contact is an important way to enhance gasliquid heat and mass transfer. Liquid film thickness at orifice is a key parameter to establish the dynamic model of gas-liquid two-phase transfer process. By considering the interaction between gas and liquid, a model for liquid film thickness around the orifice was proposed for high gas velocity flow, and the effects of factors on the liquid film thickness were clarified by discussing the interfacial stress and velocity. The comparisons between the predicted liquid film thickness and the experimental values in the case of gasliquid annular flow in pipe show that the model is very effective and widely applicable. Based on new model, the influence mechanism of gas flow rate, liquid flow rate, liquid kinematic viscosity and orifice diameter on liquid film thickness was analyzed from the perspective of fluid force. The results show that with the increase of gas flow rate, the gasliquid interface velocity and the shear force increase but the dimensionless liquid film thickness decreases. As the liquid flow rate increases, the gas flow area decreases and therefore leads to the increase of gas velocity. Although both the shear stress and the velocity at the gas-liquid interface increase, the direct increment of liquid volume eventually leads to the increase of the dimensionless liquid film. With increasing liquid phase kinematic viscosity, although the gasliquid interface shear stress increases, the gasliquid interface velocity decreases and finally leads to the increase of the liquid film thickness. At the same flow rate, with increasing orifice diameter, the shear stress and velocity of gasliquid interface decrease, and therefore lead to the increase of liquid film thickness.