On the basis of the characteristics of thin-wall injection molding technology and the principle of ultrasonic vibration, an ultrasonic-assistant system consisting of an ultrasonic generator, an ultrasonic horn, and an ultrasonic transducer is developed in this study. A simulation analysis of the injection molding of thin-wall microinjection parts is conducted using MOLDFLOW to optimize the gate, runner, primary runner, and cooling system of the mold. A cavity pressure measurement system is built for ultrasonic-assisted microinjection molding process pressure detection, the optimization of process parameters, and the improvement of product quality. The development and processing of the ultrasonic-assistant thin-wall microinjection mold are also completed by combining the ultrasonic system, thin-wall microinjection mold, and cavity pressure measurement system.