1. A computer-implemented method for filtering noise in an image, the method comprising: (a) obtaining a current image and a background image captured by an image obtaining device from a storage device; (b) adding the current image to a background list of the current image if the background list is null; (c) calculating a discrepant block between the current image and the background image if the background list is not null; (d) filtering each pixel of the discrepant block whose difference value of red, green, and blue (RGB) is less than a first preset value from the discrepant block; (e) filtering each small block of the discrepant block whose area is less than a second preset value from the discrepant block; (f) calculating a total area of each big block of the discrepant block whose area is greater than or equal to the second preset value from the discrepant block, and determining if the total area is less than a third preset value; and (g) adding the current image being filtered to the background list if the total area is less than the third preset value, or returning to block (a) if the total area is greater than or equal to the third preset value.