Electric cabin supply air system of more electric aircraft was introduced to discuss the future developing trend of environmental control system. The system architecture and main component were described to help understanding the system. Mathematical model were established to calculate the performance for external environment, ram air duct, cabin air compressor, add heat valve, check valve and ozone converter. Thermodynamics calculation is carried out for the electric cabin supply air system. According to operational scenario, design parameters of major component are determined and performance are analyzed under cruising, ground and climbing condition. The calculation results show that the component design parameters determined can meet the functional and performance requirements of air conditioning system and can be used as reference for electric cabin supply air system design.