Underwing engine nacelle is most commonly used aerodynamic layout of modern large and mediumsized aircraft. The spanwise position, chord position, vertical position, toein angle and installation angle of the engine nacellepylon relative to the wing will affect the flow field between them, then the aerodynamic characteristics of the whole aircraft are affected. The CFD numerical calculation method is used to optimize toein angle of underwing nacelle and its pylon. We compared the interference resistance of highspeed cruise and stall characteristics of lowspeed high angle of attack, researched the mechanism of negative pressure peak on the inside of the pylon, and influence of pylon with different section shapes on toein angle. The calculated results show the interference resistance and lift loss are small when the internal deflection angle is at -0.5°, 0°and 0.5°respectively, and the pylon with different profile shapes will not have great influence on the optimization results of internal deflection angle, but can reduce the negative pressure peak value inside the pylon. The conclusion in this paper has some guiding significance for hanging the external store under the upper wing.