For the change of fuel injection and discharge quantity led by the change of structural parameters for precision pair during the process of injector working, the injector simulation model was built by taking Bosch CRIN2/3 injector as the research object with AMESim software based on the theoretical analysis for the effects of fuel injector precision pair wear on fuel injection and leakage. Then the regression model between test factor and test indicator was established with the BoxBehnken design method by designing the plunger coupler gap, the needle valve gap and the needle valve lift as test factors and taking the fuel injection amount and leakage amount as test indicators, and the influences of each factor on test indicators were further analyzed. The multiobjective optimization method was used to conclude that the smallest leakage amount was obtained and the change rate of fuel injection could meet the requirements of national standards when the plunger coupler gap range was 3.00~3.07 μm, the needle valve coupler gap range was 1.50~1.77 μm, and the needle valve lift range was 0.32~0.38 mm.