Numerical simulation method of high bypass ratio separate flow exhaust system is adopted and satisfactory result has obtained. Based on the initial exhaust system, the influence of the different outlet area of external and internal bypass, the different length of the outer casing and tail cone on the aerodynamic performance have been investigated. Through analysis and investigation, this paper provides the working foundation to the design of the similar exhaust systems. The results show the existence of an optimal available pressure ratio of external bypass which makes thrust coefficient maximum, but the external bypass will be at under-expanded state when the pressure ratio is adopted. Its design is different from the single channel nozzle. Secondly, there is an optimal outlet area of nozzle which makes thrust coefficient maximum, but the nozzle will be at over-expanded state when the outlet area is adopted. It mainly depends on the pressure distribution of the outer casing and tail cone. Thirdly, the length of outer casing also influences the performance of the exhaust system.