1. A text image trimming method comprises following steps: Step 110 , obtaining text image data; gaining image data in the step 110 comprises one of the following three methods: taking the image by a DC; scanning the image by a scanner; opening an existing file that contains image data, reading the data in the file and decompressing the data with a standard algorithm; Step 120 , using straight line detection method to detect straight lines in the text image, obtaining the edges of a trimmed quadrangle; obtaining at least one candidate quadrilateral area according to the detected straight lines; Step 130 , detecting text in the image data, obtaining the coordinate of the boundary points of the text region; Step 140 , obtaining the final trimming result according to the results of steps 120 and 130 ; specifically, the step 140 comprises following steps: Step 210 , forming a set of candidate quadrangles that composed of all the quadrangles obtained in the step 120 ; Step 220 , if the set of candidate quadrangles is empty, go to step 260 , otherwise, go to step 230 ; Step 230 , popping a candidate quadrangle from the set of candidate quadrangles; Step 240 , determining if all the edge points of the text region are in the candidate quadrangle, if they are, judge the candidate quadrangle to be valid trimmed quadrangle and go to step 250 , if they are not, go to step 220 ; Step 250 , returning to the valid trimmed quadrangle; and Step 260 , returning the valid trimmed quadrangle to user for manually determining trimmed quadrangle; Step 150 , the end.