When using the implicit Monte Carlo method to simulate the thermal radiative photon transfer in materials, the radiative source particles emitted by the material should be treated carefully. In traditional sampling method for the location of radiative source particles can be regarded as uniform in a single cell, and this is reasonable for most of thermal radiative transfer problems. But it may result in unacceptable error in the problem which has a large absorption cross-section and remarkable temperature difference in a single cell. The reason for this error is discussed in this paper. A new method based on radiation energy density distribution is proposed and the sampling formula is derived. Numerical test indicates that the new method will give results much better than the old method and in agreement with the analytical results.