1. A display testing apparatus connected with an image capturing device, the apparatus comprising: a storage; an image capturing module capable of controlling the image capturing device to capture an image of a to-be-tested display and storing the captured image to the storage; a first vertex determining module capable of determining a first vertex of a test area on the captured image, comprising: a boundary pixels determining sub-module capable of determining a predetermined rectangular area which includes the first vertex, and determining boundary pixels according to RGB values of each pixel in the predetermined rectangular area; and a vertex determining sub-module capable of determining the first vertex among the determined boundary pixels according to coordinates of the boundary pixels; a test area determining module capable of determining the test area, comprising: a second vertex determining sub-module capable of lining the boundary pixels of the right side of the first vertex through the first vertex to obtain a first border, and lining the boundary pixels of the left side of the first vertex through the first vertex to obtain a second border; the second vertex determining sub-module further capable of reading pixels of the captured image from the first vertex until a first boundary pixel is found along a first direction, determining a first line parallel to the second border through the first boundary pixel to obtain a third border of the test area, and obtaining a second vertex which is an intersection between the third border and the first border; and a remainder vertices determining sub-module capable of reading pixels of the captured image from the second vertex until a first boundary pixel is found along a second direction, determining a second line parallel to the first border through the first boundary pixel to obtain a fourth border of the test area, obtaining a third vertex which is an intersection between the fourth border and the second border, obtaining a fourth vertex which is an intersection between the fourth border and the third border, and defining the test area according to the first vertex, the second vertex, the third vertex, and the fourth vertex; and a testing module is configured for testing parameters of the display according to the test area.