1. A method of providing augmented reality based on marker tracking technology, the method comprising: capturing an image by an image capturing unit; determining whether a quadrangle is present in the image, wherein the step of determining whether the quadrangle is present in the image comprises: converting the image into a grayscale image; performing a mask computation on the grayscale image to acquire a plurality of edge pixels in the image and calculating a corresponding direction of each of the edge pixels; defining parts of the edge pixels as a plurality of skeleton pixels according to relative positions of the edge pixels; scanning the image according in a specific order, and once one of the skeleton pixels is scanned, determining whether a scanning direction is changed five times in a process of continuously acquiring another one of the skeleton pixels or another one of the edge pixels; and when the scanning direction is changed five times in the process of continuously acquiring another one of the skeleton pixels or another one of the edge pixels, determining the quadrangle to be present in the image and calculating four vertex coordinates of the quadrangle; when the quadrangle is found in the image, determining whether the quadrangle is a marker in accordance with a marker definition; when the quadrangle is determined to be the marker, identifying an order of four vertex coordinates of a marker image and identifying an identity of the marker; determining a rotation state of the marker according to the vertex coordinates of the marker image and calculating a relative displacement between the marker and the image capturing unit; and combining a three-dimensional object to the image according to the relative displacement, the rotation state, and the identity of the marker to generate an augmented reality image.