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; and the image processing unit comprises: an image processing module; a grey preprocessing module; a denoising module; and a contrast enhancing module; wherein the image processing module operates to mirror the image transmitted from the CMOS image sensor to the image processing unit; the grey preprocessing module operates to average grey values of pixel points in a vein area in the image processed by the image processing module, then substrate a resulting average from each of the grey values of the pixel points to achieve the grey preprocessing; the denoising module operates to eliminate Gaussian noise and salt and pepper noise in the image after grey preprocessing based on a grey K-Nearest Neighbor (KNN) Algorithm; and the contrast enhancing module operates to enhance a contrast of the image after denoising using a threshold segmentation method so as to highlight the vein area; the threshold segmentation method sets a grey threshold, and places pixel points having higher grey values than the grey threshold into one class, and pixel points having lower grey values than the grey threshold into another class.