Abstract: When the axial section of internal strain gauge balance is applied large load, especially large moment load, the maximum stress of its flex beam often becomes the main factor limiting the maximum load capacity of the balance. This paper introduces the development of a large moment internal strain gauge balance. The flex beam and measuring beam of the axial section are optimized and improved by using finite element simulation analysis. The shape of the flex beam is improved from traditional constant cross section to variable cross section. The thickness of the middle part of the flex beam is reduced, and the thickness of both ends is increased. The maximum stress on the flex beam is reduced on the basis of maintaining the same axial stiffness of the balance. The axial force measuring beam adopts variable section structure, which reduces the strain gradient on the measuring beam. The finite element analysis shows that the stress distribution on the variable-section flex beam is more uniform than that of the traditional flex beam, and the maximum stress at the root of the variable-section flex beam is reduced by more than 20%. The strain gradient on the measuring beam with variable cross section is reduced by 79% compared with that on the beam with constant cross section. The calibration results of the balance are in good agreement with the finite element analysis results, and the wind tunnel force test also shows that the balance has good stability.