The invention discloses a multi-grade waste heat energy cascade recycling system for a diesel engine, and particularly relates to a combined-type diesel engine waste heat energy recycling system. The waste heat energy recycling system is structurally characterized in that a high-temperature Rankine cycle system is composed of a working medium pump, a thermoelectric generator, an EGR heat exchanger, an exhaust heat exchanger, a steam turbine, a condenser and the like of a high-temperature system; a low-temperature Rankine cycle system is composed of a working medium pump, an EGR heat exchanger, an exhaust heat exchanger, the cylinder body cooling water heat exchanger, the supercharged air heat exchanger, a steam turbine, a condenser and the like of a low-temperature system. A middle electrical machine is arranged between a turbine motor and a supercharger; the waste heat of cooling water, supercharged air, exhausted gas, EGR recirculated waste gas and the like are utilized, and through the high-temperature and low-temperature organic Rankine cycles, thermoelectric power generation, turbine direct power generation and the like, heat exchange can be carried out between various kinds of waste heat generated by the diesel engine and working media sequentially according to the grade. Due to arrangement of the middle electrical machine, the expansion ratio of the exhausted gas can be improved, and excess pressure energy of the exhausted gas can be directly recycled. Full recycling of multi-grade waste heat energy of an internal combustion engine can be achieved, irreversible losses of the system are reduced, the fuel economical efficiency is effectively improved, and therefore the aims of saving energy and reducing emissions are achieved.