A method for calibrating a device for monitoring the curvature of a stiffener (18) of a flexible sea line: A monitoring device has a deformable rod (26) having a central axis (C) and at least three optical sensors (29, 30, 31) maintained pressed against the perimeter of the rod (26). The method includes the steps, for different consecutive orientations of bending planes around the central axis; bending the rod (26) according to the same curvature radius; measuring the deformation of the sensors (29, 30, 31) during the bending; using the measured deformations to extrapolate a sine function of the deformation for each sensor according to the orientation of the bending plane; calculating error-correction coefficients according to the angular shift between the extrapolated sine functions and according to the amplitude of the extrapolated sine curves.