The invention provides an optimal control method of plug-and-play urea injection system OBD monitoring. The optimal control method of plug-and-play urea injection system OBD monitoring comprises the steps that under the condition that the plug-and-play system cannot receive the work state of a pump or process pumping pressure, through correction of fault code information of a DCU end and a trigger requirement of an ECU end, a SPN1 is additionally arranged, the definitely diagnosed fault that the urea pump is not in the injection state and the system always tries voltage buildup is processed into the measure that irrelevant lamp turning on is conducted and an ECU is triggered not to conduct efficiency diagnosis, the other SPN2 is additionally arranged to process the fault that the urea pump breaks down, stops and limits torque after 50 hours into the measure that relevant lamp turning on is conducted, and the ECU is triggered not to conduct efficiency diagnosis, and then requirements of laws and regulations are better met.