The flexible beam is two bidirectional flexure hinge beam，is widely used as main component of wind tunnel cassette strain gauge balance，its stiffness performance directly affects the accuracy of wind tunnel cassette strain gauge balance，and it is an important guarantee for accuracy of test data of engine test bench，whereas flexure hinge design is the key of flexible beam design. Firstly，four kinds of typical flexure hinges are analyzed by finite element method，a flexure hinge which has good integrated stiffness is selected. Secondly，for selected hinge style， the influence of three key structural parameters on hinge stiffness performance is researched. Finally，basis on above research，five kinds of min-thickness flexible beams are designed and analyzed by finite element method，and their stiffness performance are validated by loading test. The result shows the hinge that has b=40mm，R=50mm and t=40mm can get best stiffness performance，in addition，there is a good agreement between calculation and test (the maximum error is less than 5%)，this indicate it is feasible and reliable that finite element method is used to design and optimization of flexible beam.