The coupling duct characteristics used in Laser Diode Array (LDA) pumped slab amplifiers was investigated by ray tracing. The ideal reflective characters were analyzed at first. And then,the coupling ducts with different parameters were simulated to prove the relation between coupling duct reflection and structure parameters,which showed that ducts had its own loss and the relations between duct reflectivity and its structure parameters were obtained. The concept of duct side wall equivalent times of reflection was also put forward,which could be used to calculate the reflection loss on the side wall. After ray tracing and numerical fitting,an approximate mathematical expression was gotten,which could be well used to show the reflection situation of duct. Finally,an experiment about practical project design was done to prove the good precision of the simulation.