Liquid cooling system is used to cool electronic equipment，and the thermal design is the key of liquid cooling system design．A small liquid cooling system which cooling power could be adjusted was designed to cool electronic equipment．Variable orifice and cold plate of heat producer were installed in parallel to adjust cooling flow．Thermal performances of liquid cooling system’s components were analyzed，and simulation models of the components and the liquid cooling system were built based on AMESim．The liquid cooling system was simulated under different work condition to verify feasibility of the design．It is shown that the designed small liquid cooling system can satisfy requirement，and AMESim is a convenience tool to design and simulate the liquid cooling system.