The modulation on radiation characteristics of random laser caused by photonic crystals(PCs) was studied, and its dependence on the size and arrangement of the crystal grains was analyzed. For a random medium, if it is covered by different photonic crystals on its upper and lower surfaces respectively, its output characteristics should be different. The results showed that the grain size of the PCs has a great effect on the radiation characteristics of the system. The PC with a suitable grain size plays a better role in trapping the energy in the system and it can modulate the laser modes effectively. It leads the light to oscillate back and forth, enhancing the interaction between the random gain medium and the light to achieve a greater amplification, and conseguently reducins the lasing threshold. It also controls the spontaneous emission and leads it to the target frequency in the emission spectra. However, if crystal grain size daeznot match the structure of random gain medium, the laser modes could not be modulated as we wish and the energy of the light field could not be localized effectively in the system, thus the laser system has a higher lasing threshold. The arrangement of crystal grains should affect the output characteristics also. In short, there exists an optimum PC/ random medium combination for lasing, for which the lasing threshold reaches a minimum.