In the aerodynamic design of compression systems, blade geometric adjustments are routinely employed to accommodate varying operational conditions. Traditional techniques, such as adjustable and variable-camber blades, only partially replicate the effects of geometric modifications due to limitations in global rotation mechanisms and restricted adjustment ranges, which impede stable and efficient performance under complex conditions. Emerging deformable materials now allow for independent spanwise blade deformation, offering a novel strategy for continuous, rapid, multi-degree-of-freedom adjustment. However, a lack of systematic methods to guide deformation patterns and material development remains a challenge. This study introduces a chord-length-preserving flexible deformation hypothesis for a typical axial flow fan and establishes a blade design methodology under constant chord constraints to mitigate efficiency losses at off-design back pressure and near-surge conditions. A NURBS-based parametric model, integrated with multi-objective optimization algorithms and CFD simulations, is employed to systematically analyze how flexible deformation of rotor and stator blades influences flow characteristics and efficiency at specific back pressure points. Results reveal improvements of 3.50% in adiabatic efficiency (with a 7.50% flow reduction) and 8.12% near-surge efficiency (with a 9.50% flow reduction), validating the approach and its potential for adaptive fan operation in complex environments.