A method for designing a predefined wind turbine or water turbine flexible blade, the flexible blade having a known geometrical profile: a known span as well as a thickness and a chord that are known and variable in the spanwise direction, a pitch angle at rest, the flexible blade being designed to be flexible at least according to the chord, for passively regulating the pitch angle of the wind turbine or water turbine in operation, the method comprising the following steps: a) receiving the known geometrical profile, b) determining, for said geometrical profile applied to a reference rigid blade, a change of the optimum pitch angle of the rigid blade θ 0 opt rigid as a function of the specific speed λ equal to the ratio between the speed considered at the end of the blade and the incident fluid speed, for a speed regime of the fluid flowing around the reference rigid blade, c) determining the local behavior of the flexible blade, deforming under the effect, on the one hand, of the aerodynamic loading of the fluid circulating around flexible blade and, on the other hand, of the centrifugal force exerted on the flexible blade in rotation, and determining local ratios relating to the aerodynamic loading and to the centrifugal force exerted on the flexible blade, d) determining local values of bending modulus B and mass density ρ, using the local ratios and the behavior of the flexible blade determined in step c) so that the change of a effective pitch angle θ 0 eff of the flexible blade, conferred by the flexibility at least according to the chord of the flexible blade, as a function of the specific speed λ, corresponds to the change determined in the previous step b), and e) restituting an information relating to the choice of the material, determined from the local values of bending modulus B and mass density ρ calculated in step d) and from the geometrical profile received in step a). a) receiving the known geometrical profile, b) determining, for said geometrical profile applied to a reference rigid blade, a change of the optimum pitch angle of the rigid blade θ 0 opt rigid as a function of the specific speed λ equal to the ratio between the speed considered at the end of the blade and the incident fluid speed, for a speed regime of the fluid flowing around the reference rigid blade, c) determining the local behavior of the flexible blade, deforming under the effect, on the one hand, of the aerodynamic loading of the fluid circulating around flexible blade and, on the other hand, of the centrifugal force exerted on the flexible blade in rotation, and determining local ratios relating to the aerodynamic loading and to the centrifugal force exerted on the flexible blade, d) determining local values of bending modulus B and mass density ρ, using the local ratios and the behavior of the flexible blade determined in step c) so that the change of a effective pitch angle θ 0 eff of the flexible blade, conferred by the flexibility at least according to the chord of the flexible blade, as a function of the specific speed λ, corresponds to the change determined in the previous step b), and e) restituting an information relating to the choice of the material, determined from the local values of bending modulus B and mass density ρ calculated in step d) and from the geometrical profile received in step a).