1. A method for filtering image noise out, comprising: acquiring a YUV image, and performing image noise filtering process to an unfiltered pixel in a color channel in the image as follows; judging the pixel currently detected in accordance with all the pixels in a preset N*M matrix centering on the pixel currently detected; if the pixel currently detected is in a flat region, performing the following steps: step S 1 , calculating and outputting a filtering strength parameter which is corresponding to the pixel currently detected in accordance with a Y-component of all the pixels in the N*M matrix; step S 2 , filtering the pixel currently detected in accordance with the filtering strength parameter; step S 3 , accomplishing the image noise filtering process performed to the pixel in the color channel; wherein, N≧3, M≧3, and both N and M are odd numbers; wherein, the N*M matrix is a filtering template used in the method for filtering image noise out; wherein, if the pixel currently detected is not in a flat region in accordance with all the pixels in the preset N*M matrix centering on the pixel currently detected, performing the step as follows: performing an average filtering to the pixel currently detected, and then accomplishing the image noise filtering process performed to the pixel in the color channel; wherein judging whether the pixel currently detected is in a flat region comprises: calculating a difference value in the maximum range of the pixel corresponding to the pixel currently detected in accordance with all the pixels in the preset N*M matrix, and judging whether the difference value in the maximum range is less than a preset first threshold, and then outputting a corresponding first judgment result; calculating a variance value of all the pixels in the preset N*M matrix, and judging whether the variance value is less than a preset second threshold, and then outputting a corresponding second judgment result; calculating and judging whether maximum distance between values of filtered pixels in first row of a preset 3*M matrix is less than a preset third threshold value, and then outputting a corresponding third judgment result; when judging whether the pixel currently detected is in a flat region, also judging the following in accordance with the first judgment result, the second judgment result and the third judgment result: if the difference value in the maximum range is less than the first threshold, and the variance value is less than the second threshold, and the maximum distance is less than the third threshold, it is determined that the pixel currently detected is in a flat region; and otherwise, it is determined that the pixel currently detected is not in a flat region.