Vehicle-mounted SCR metering and ejecting system taking NOx in exhaust gas as variable

The invention relates to a vehicle-mounted SCR metering and ejecting control system for a urea reducing agent, which takes NOx in exhaust gas as variable. The system is characterized in that a precision NOx sensor is arranged at the inlet end of an SCR rear processor, an intelligent PID controller and a self-programming computer program are adopted, and a feedback signal is generated on a NOx controlling unit; the flow of an electromagnetic metering diaphragm pump is controlled through a NOx signal, so that the filtered urea reducing agent from a urea reducing agent tank is output according toa certain dosage and flows into a gas liquid mixing unit with two cavities through a one-way valve; the filtered compressed air after pressure adjustment also flows into the gas liquid mixing unit wiht the two cavities; and the mixed gas of the urea reducing agent and the air is injected into the front end of the SCR rear processor through a spraying nozzle, and the NH3 decomposed from the urea reducing agent reacts with NOx in the exhaust gas on the surface of SCR catalyst. The innocent treatment is accomplished, the loop is automatically controlled, and the vehicle-mounted ECU and the vehicle-mounted DCU are replaced, and therefore, the cost is saved.