Abstract: Drilling liquid-gas separator is an important well control device in the process of drilling. If it is not operated appropriately, the combustible gas will flow to the platform and consequently threaten personnel and equipment safety. At present, the maximum flow rate during drilling blowout is estimated mainly based on experience, and the separators used in the drilling are mainly in reference to the standards related to oil and gas production separators. To solve these problems, a method to estimate the maximum flow rate of gas and liquid during the well control of drilling was established according to multi-phase flow theories. After the structure and working principle of liquid-gas separator were studied, ID and height of separator, size of connection pipe, height of liquid seal section, property of drilling fluid, gas and liquid flow rate inside the separator and erosion of separator were analyzed to determine the key factors resulting in separator failure during the well control of drilling. It was pointed out that there are four forms of separator failure, i.e., carrying and discharge of drilling fluid with the gas, discharge of gas through the drainage pipe, discharge of drilling fluid through the vent pipe, and erosion of separator. Finally, based on blowout conditions, a multi-factor fusion based separator evaluation method was formulated and the measures to transform old separators were prepared on the basis of case study, so as to ensure the reasonable and safe operation of the separators during the well control of drilling.