This paper studies that a twin-engines light general aircraft completed modification, which changed the aerodynamic characteristics, so it also impacted the flight performance and control-stability. As an example in this paper, the aerodynamic characteristics of an aircraft with external pods were calculated using CFD，then the influence of the fight performance after modification was calculated and the control-stability was analyzed. The result show that, the modification has some influence to the flight performance, and less effect to the control-stability，also it can guide the aircraft mounting pods, meanwhile it has a certain reference value of airworthiness certification and flight test.