The invention relates to an organic Rankine cycle diesel engine tail gas and waste heat utilization system using a two-stage single-screw expanding machine, belonging to the field of energy conservation and emission reduction, and mainly comprising a gas compressor, an engine, a turbomachine, a tube bundle type evaporator, a single-screw expanding machine I, a magnetic powder dynamometer I, a single-screw expanding machine II, a magnetic powder dynamometer II, a plate type condenser, a liquid storage tank, a working medium pump, an intermediate heater, a cooling tower, an adjustable flow pump, a flow divider valve I of an expanding machine outlet, a flow divider valve II of the expanding machine outlet, a flow divider valve III of the expanding machine outlet, an exhaust control valve I, an exhaust control valve II, a turbomachine and compressor coupling shaft, a coupling shaft I, a coupling shaft II and various connecting pipelines. According to the conditions of different rotating speeds and different loads of a diesel engine, by utilizing the invention, the opening and closing of the flow divider valves of the expanding machine outlet and the exhaust control valves are adjusted, waste heat recoveries at different levels are realized and the energy taken away by the tail gas of the diesel engine is furthest recovered and utilized.