Abstract: The composition of working fluids has considerable influence on the performance of the mixed refrigerant cycle (MRC) of gas liquefaction system and is also the key for optimization of the gas liquefaction process and cycle performance. During the adjustment and operation of actual system, it is necessary to analyze the working mixture composition quantitatively. In this paper, a composition testing method has been established, which is based on gas chromatography (GC) for the mixed refrigerant used in a small-scale pre-cooling cycle gas liquefaction system. It is shown that the developed method has the advantages of rapidity, simpleness, convenience and accuracy in determinating the composition of multi-component working fluids and can be effectively used to optimize the MRC gas liquefaction system.