The invention provides a combined cycle thermoelectric conversion system utilizing multi-grade waste heat of an internal combustion engine. According to the system, combined organic Rankine cycle is formed by high-temperature grade cycle and low-temperature grade cycle, heat is supplied by exhaust waste heat of the internal combustion engine, supercharged and mid-cooling waste heat, exhaust gas recirculation waste heat and cooling water waste heat, a cooling quantity is provided by cold wind, and the system comprises a high-temperature grade working medium pump, a high-temperature grade heat regenerator, a high-temperature grade exhaust heater, a high-temperature grade EGR (exhaust gas recirculation) heater, a high-temperature grade expansion engine, a high-temperature grade condenser, a low-temperature grade working medium pump, a low-temperature grade heat regenerator, a low-temperature grade cooling water heater, a low-temperature grade supercharged air heater, a low-temperature grade exhaust heater, a low-temperature grade EGR heater, a low-temperature grade expansion engine, a low-temperature grade condenser, a low-temperature grade condenser fan and a power generator. According to the combined cycle thermoelectric conversion system utilizing multi-grade waste heat of the internal combustion engine, comprehensive and efficient utilization of multi-grade waste heat of the internal combustion engine can be achieved, further, irreversible loss is smaller when both the high temperature and the low temperature adopt supercritical expanding cycle, and the integral efficiency of the internal combustion engine can be effectively improved.