Expander-compressor is the core equipment in the ethylene plant. Expander-compressor unit runs at high speed and low temperature, so it has high requirements for design, material and bearing system. At present, expander-compressor is highly dependent on imported units. Expander-compressor's performance test is an important method to evaluate the performance of the expander-compressor unit, but there is no relevant mature standard on this topic to refer to. In this paper, the expander-compressor's performance test scheme is introduced, including the design of test speed, test medium, test conditions and the conversion of the test results based on the machine characteristic, and a program used for the test result validation is developed based on the Similitude Principle. It shows that the test isentropic efficiency is 82.55%, which is higher than the design isentropic value, and the performance of this expander-compressor can meet the evaluation criteria specified in the standard. In conclusion, the designed performance test scheme and test result validation program using Similitude Principle can be effectively applied to the engineering field.