For the design of gas-liquid separator,the design specification HG/T 20570.8-95 has been used for calculation,and the design of horizontal separator has to adopt the way of trial and error,which involves checking the diagrams and increases the workload without any line,so in order to save the time of calculation and to improve the accuracy of the calculation,the form of the calculation of the horizontal separator is compiled.Based on this,the form of the table and the process of deriving the bowed area formula are given,and the calculation process of a horizontal separator for a low-temperature methanol washing plant is introduced in conjunction with a project that has already been implemented in order to gain a detailed understanding of the functions of each table and matters to be taken into account in the calculation process.The process of calculating each parameter in the calculation module prepared with Excel sheets and the formulas used in them are explained.