According to the statistics about civil transportation aircraft Environmental Control system (ECS), the three-wheel high pressure water separation system (HPWS) and low pressure water separation system (LPWS) are the most common choices for the 150-seat airliners. Although the former has become the mainstream configuration for air conditioning pack, the latter is still used on Boeing 737-600/700. In order to compare the two configurations and choose the better one for a specific aircraft, simulation and analysis are done. The cabin heat load is calculated at first in order to calculate required engine bleed air mass flow. Then a specific aircraft is defined so that required structural dimensions and cabin comfort indexes can be obtained based on Airbus 320. Thirdly, the component models are built by Matlab/Simulink according to the fundamental knowledge of heat transfer and aerodynamics, the working principles and mechanical dimensions of the components, the ambient environmental parameters and some data from Airbus 320. Consequently, the complete system model can be assembled. After confirming the validity of the model by checking the required ram air mass flow and temperature deviation of the state points referred to Airbus 320, the simulation model is used to do analyze the specific aircraft. Finally, through comparing the different values of ram air mass flow and turbine expansion ratio, as well as the system mass, economic cost and reliability, the better configuration is selected. It can be summarized that the three-wheel LPWS requires less ram air mass flow (0.012kg/s) and a little lower expansion ratio (0.02) than the HPWS, and it also has lower weight (63% of HPWS), lower (83% of HPWS) cost and higher reliability (140% of HPWS), thus it is the suitable configuration for the specific aircraft.