A liquid-containing gas solid fluidized bed reactor (FBR) is the core equipment for the ethylene polymerization process operated in the condensed mode. However, the thermal stability characteristics of the liquid containing gas solid FBR is far from understood. In this work, a novel model based on the combination of a two-phase model and a liquid evaporation model is developed to describe the thermal stability of a liquid containing FBR. The effects of catalyst activation energy and condensed agent content on the thermal stability and multiplicity of the ethylene polymerization in a liquid-containing FBR is studied through the homotopy continuation method. With the increase of activation energy and increase of condensed agent content, the stable branch is reduced and the steady state multiplicity appears. Furthermore, the dynamic behaviors of the liquid containing FBR at the Hopf bifurcation points of the low-temperature branch are characterized. It is shown that a negative perturbation of the catalyst feed rate leads to the state variables oscillating and evolving into a stable state, while a positive perturbation of the catalyst feed rate leads to running away of the state variables. Results from this work provides a guidance to the industrial operation of liquid-containing FBRs and the design of the corresponding control systems.