Aiming at the problem of poor injection stability and low precision of quantity control caused by poor monotonicity of MAP for common rail injector under the small fuel quantity condition, an AMESim simulation model of injector was established to analyze the motion state of solenoid valve and needle valve under the small fuel quantity condition. It was concluded that the reason of MAP poor monotonicity was that the needle valve opened before the solenoid valve completed collision with the electromagnet. With the increase of the pulse width, the speed of the solenoid valve changed suddenly when the solenoid valve collided with the electromagnet by its own inertia, which led to the decrease for the closing delay of solenoid valve and needle valve and a consequent decrease in circulating injection volume. Therefore, the MAP of injector with small fuel quantity was optimized by reducing the opening time of solenoid valve and increasing the opening delay of needle valve. Finally, the scheme of increasing the diameter of sealing conical surface from needle valve was used, which not only optimized the injection performance of small fuel quantity condition, but also decreased the injection duration of rated load.