Abstract: In geotechnical and mining engineering, numerous particle matters are involved in scientific and technical problems. Recognition and quantification of the structure and distribution of force chain networks using photoelastic experiments, for instance, are significantly important in understanding the internal mechanism of mesoscopic mechanics and studying the macroscopic mechanical behavior. Based on the algorithm of the mean square value of color gradient (G²), the correlations of G² with the contact force (F) of different sizes of round and square particles were established and combined with digital image processing technology. A method was proposed for identifying particles in images and distinguishing square particles from round ones, and force chain structures and force chain distributions were obtained in photoelastic images. Using fully mechanized caving mining as an example, the proposed method was verified, and it elaborated the formation and characterization of mining-induced stress in a top-coal caving mining face. The study shows that F monotonically increases with increased G², and larger particle sizes correspond to a faster growth of F. The contact forces of singular particle distribute primarily between 0.5 and 1.5 times average contact force. Also, the average contact forces in square particles and Φ12 mm circular particles are higher and mainly occur in strong force chains, whereas average contact forces in Φ10 mm and Φ8 mm circular particles are lower and primarily occur in weak force chains. In top-coal caving mining, force chains in top-coal and overburden strata are mainly displayed in tree-like forms. Strong force chains, which extend in a vertical direction, transmit majority overburden loads, whereas lateral-developed weak force chains play a role in supporting strong force chains. In the vicinity of the top-coal outlets, because of the lateral movement of particles toward the mined area, the weak force chains in top coal disappear, resulting in the strong force chains becoming weaker or completely disappearing.