In order to recover the multi-grades waste heat of Diesel Engine with low exergy loss, a combined trans-critical Organic Rankine Cycle(CORC) was presented in this paper. This combined cycle includes two parts, one is called high T stage cycle(HORC) and used to recover engine's exhaust and EGR heat with high temperature, and the other one is called low T stage cycle(LORC) and used to recover engine's coolant heat, compressed air heat, residual exhaust and EGR heat. In this paper, various systems with various high temperature working fluids in HORC were calculated by analyzing these system parameters such as system thermal efficiency, recovered power, exergy efficiency and global efficiency of DE-CORC as the increase of HORC's high working pressure. The results indicted that there is an optimum HORC's high working pressure at which the system thermal efficiency reach to the maximum value. Moreover, system using toluene as HORC's working fluid had a better system thermal efficiency and exergy efficiency, and achieved a 6.86% increase of global efficiency compared with the diesel engine without any waste heat recovery system.