A dynamic photoelastic imaging system is presented and employed to study the directivity of ultrasonic scattering field. Focusing on the cylindrical cavity, photoelastic photograph displays the scattering field, and the directivity patterns of different distances can be plotted by the gray level per pixel. Compared with the simulation results by finite element method (FEM), dynamic photoelastic technique shows more reliable directivity patterns which are in close proximity to the practical scattering field.