Based on considering the requirements of the aircraft stability and control， aerodynamic design， structural design and cargo loading， we can define the aircraft CG(centre of gravity) envelope. In the practice of aircraft design, it is necessary to improve the CG limit lines. Firstly, some restrictions are made on the aircraft CG envelop to satisfy the actual operation so as to ensure flight safety. Secondly, the structural loads are reduced so as to achieve a minimum structural weight. Thirdly, the maximum takeoff weight of the aircraft is increased further, and the performance potential of the aircraft is taped so as to improve the competitiveness. Domestic and foreign data of the aircraft weight and balance, especially of Airbus aircraft families and Boeing aircraft families, were analyzed and the main parameters impacting the CG variety were put forward. Combining all of the requirements of weights, structures, strengths and landing gears, the necessary conditions for CG limit modification, a modification approach and operating procedure were given, which could be used as a reference for novel aircraft design. For shaping the aircraft CG limit line, design iteration is an important part of the process. It is also an iterative process to modify the center of gravity envelope until a reasonable optimal solution that meets many requirements is obtained.