Graded-index materials are effective to control thermo-optical systems, due to its special physical characteristics. And quantitatively calculation for energy transfer process is valuable for the research of graded-index materials. Thus, reliable numerical method is required. Lattice Boltzmann method (LBM) is a mesoscopic numerical method with unique advantages on coupling muti-physical field. Here, we provide a LBM to couple radiation and conduction within graded-index medium under a unified framework. Through several 1-D cases, the great consistence between present LBM and references indicates that LBM has excellent availability and accuracy. Furthermore, LBM is expanded to coupled 2-D problem. The present LBM is expected to simulate coupled radiation-conduction heat transfer in participating medium with graded-index effectively.