A miniature air compressor includes a motor, a compression cylinder, a lower valve seat, an upper valve seat, and a noise suppression cylinder. Thus, the miniature air compressor may provide a stable pressurized air flow whose noise and impurity may be eliminated by the noise suppression cylinder, thereby enhancing the usage effect of the injection pen device. In addition, the miniature air compressor has a simple construction, may be assembled and amended easily and conveniently, and may reduce the cost of production.