The invention relates to a system capable of utilizing smoke waste heat in a gradient mode for assisting in removing SO3 and improving dust removing efficiency. The system comprises a boiler, a denitration device, an air preheater, an electric dust remover, an induced draft fan, a desulfurizing absorption tower, a chimney, low-pressure heaters, high-pressure heaters, heat exchangers, regulating valves and flow pumps, wherein the denitration device, the air preheater, the electric dust remover, the induced draft fan, the desulfurizing absorption tower and the chimney are connected with a flue at the tail of the boiler in sequence; the low-pressure heaters, the high-pressure heaters and the heat exchangers are arranged on bypasses; the regulating valves and the flow pumps are arranged on pipes and used for controlling flow; the air preheater is connected with the first bypass in parallel, the second bypass is arranged between the air preheater and the electric dust remover, and the third bypass is arranged between the induced draft fan and the desulfurizing absorption tower. The system follows the energy gradient utilization principle, smoke of the boiler is divided to three parts for waste heat utilization, and the smoke waste heat is deeply recycled; besides, the specific resistance of dust in the smoke can be changed, the electric dust removing efficiency can be improved, and the SO3 concentration in the smoke can be reduced.