Abstract: The determination of plane photoelastic stress for three-point bending beams is widely recognized as a difficult teaching topic in the experimental mechanics curriculum. To address this, this paper introduced the cubic spline interpolation method and obtained the full-field data of the arithmetic lines and inclination lines on the computational surface and auxiliary surfaces within the beam. Furthermore, the shear stress difference method was adpoted to determine the experimental solution of sectional stress. By comparing the results with numerical and theoretical solutions, the stress distribution laws were in good agreement, which verified the effectiveness of the cubic spline interpolation method in solving the stress of three-point bending beams. The introduction of this method had facilitated the efficient resolution of teaching challenges, fostering students' innovative thinking and practical skills in addressing interdisciplinary problems.