Pneumatic design method for separation type exhaust jet pipe of high bypass ratio engine, which is characterized in that: the method adopts parametrization design, a straight line segment form composes a pneumatic shape of the jet pipe, circular arcs are used for smoothly connecting among the straight line segments. A concrete flow is that: step 1, defining an initial parameter of the pneumatic design of the jet pipe, and assigning; step 2, solving the other parameter according with a geometric relation of the jet pipe shape and establishing a mathematics model; step 3, generating the jet pipe shape controlled by the step 1 and the step 2; step 4, examining the pneumatic shape whether reaching to a required pneumatic performance - if yes, finishing the design flow; if no, returning to thestep 1 and restarting the flow. The design method can design an inner-outer bypass jet-pipe flow scheme rapidly, check the scheme whether practicably reduce the design period obviously, and ensure a simple processing method.