The preparation of photoelastic coatings is a critical step in photoelastic experiments, as the coating formulation and manufacturing process strongly influence the resulting photomechanical properties and measurement accuracy. This study optimizes the processing parameters of photoelastic coatings, focusing on how the mass ratios of epoxy resin, plasticizer, and curing agent, together with the stirring temperature during the addition of the curing agent, affect the material"s fringe constant. The results show that the fringe constant first decreases and then increases with rising stirring temperature, reaching a minimum at 60 °C. Insufficient curing agent prevents complete coating curing, while an excess leads to an increased fringe constant; the optimal curing agent ratio is determined to be WG = 0.08. Similarly, an excessively low plasticizer ratio impairs coating formability, whereas too much plasticizer reduces the fringe constant, with the optimal plasticizer ratio identified as WZ = 0.08. Comprehensive analysis indicates that the coating attains the lowest fringe constant (15.38 N/mm) and the best photoelastic effect at a stirring temperature of 60 °C and an epoxy resin : plasticizer : curing agent mass ratio of 100:8:8. Using this optimized coating, a reflection photoelastic experiment was conducted. Combined with a ten-step phase-shifting method, a principal stress calculation approach was proposed to enable full-field stress visualization. Validation against numerical simulations yields average errors of 4.4%, 5.9%, and 6.8% for the maximum principal stress, minimum principal stress, and shear stress, respectively. This work provides robust technical support for the development and practical application of reflection photoelastic testing.