When an object was subjected an impact loading, stress wave was produced in the object. Studying the regularity of stress-wave propagation was significant to the study of objects subjected to impact loading. When stress wave travelled in the object, principal stress on free boundary was useful to theoretical analysis and calculation. In this article, a new kind of dynamic photoelastic apparatus was used. Isochromatic and isoclinic of the object subjected to impact loading could be obtained combining dynamic photoelastic experiment and related test equipment. By analyzing the isoclinic, there would be a conclusion that the angle between the isoclinic and the free boundary was not 0°or 90°. So the values of the two principal stress on the boundary were all not 0. The result obtained from the electrometric method came to the same conclusion. Analysis showed the result of dynamic photoelastic method was compatible with the result of electrometric method. So the method in this article was feasible and accurate.