Abstract: Gas-liquid two-phase flow is widely found in reactor cores and equipments such as steam generators. The bubble kinetic behaviors, such as bubble coalescence and breakup, directly affect the thermal-hydraulic performance of the related equipment. Bubble coalescence can affect flow characteristics such as turbulence intensity, velocity distribution, and system pressure drop. Bubble coalescence can also lead to the occurrence of flow instability, which must be avoided during reactor operation. It is necessary to consider the influence of bubble coalescence and other behaviors in the process of designing heat exchange equipment and systems. The population balance model (PBM) is an important method to deal with the equilibrium problem of multi-component particles in multi-phase flow systems. The bubble coalescence kernel function is one of the main kernel functions of PBM, which is used to describe the probability that two or more bubbles will coalesce into a larger bubble during collision, and its accuracy directly affects the study of bubble kinetic behavior and the prediction of thermal-hydraulic performance under the Euler-Euler frameworks. The film drainage model is the most widely used coalescence efficiency model, which is an important part of the coalescence kernel function. The definition of bubble contact time and the quantification of initial liquid film thickness have not been clearly pointed out in previous studies. The influence of bubble diameter, and continuous phase viscosity on the initial liquid film thickness and liquid drainage time is still unclear. In this paper, the 2D numerical simulation of two coaxial bubbles coalescence process was carried out by the volume of fluid method (VOF) combined with the continuous surface force model (CSF) by ANSYS_FLUENT, and the drainage time of the bubble coalescence process under different viscosities was quantitatively obtained according to the simulation results. The contact moment when the bubbles started to discharge was quantitatively described, and the relationship between the initial film thickness and the bubble diameter, deformation ratio, and viscosity was meticulously discussed. The applicability and accuracy of the four existing drainage time models in the literature were systematically evaluated. The moment corresponding to the turning point of the relative velocity of the two bubble films was defined as the contact moment. It is found that the initial liquid film thickness and drainage time are affected by the bubble diameter and liquid viscosity. The results show that the definition of the contact moment and the comprehensive consideration and evaluation of the effects of viscous and inertial forces are of great significance to improve the accuracy of the drainage time model, extend its scope of application, and thus improve the calculation accuracy of the PBM method.