1. A portable vein projector, comprising: an LED light source; a CMOS image sensor; a semi-transparent and semi-reflecting mirror; a micro-projector; and an image processing unit; wherein in use, the LED light source projects near-infrared light on a skin surface area; the near-infrared light is reflected by the skin surface area, and part of reflected light penetrates through the semi-transparent and semi-reflecting mirror, enters into the CMOS image sensor to form an image in the CMOS image sensor; the image is transmitted from the CMOS image sensor to the image processing unit where the image is processed, and processed image is transmitted to the micro-projector; the micro-projector outputs visible light; the visible light is reflected by the semi-transparent and semi-reflecting mirror and is imaged on the skin surface area to yield a vein image.