1. A computer-implemented method for filtering noise in an image using a computer comprising a processor, the method comprising execution of the steps comprising: (a) obtaining a current image and a background image captured by an image obtaining device from a storage device using the processor; (b) adding the current image to a background list of the current image if the background list is null using the processor; (c) calculating a discrepant block between the current image and the background image if the background list is not null using the processor; (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 using the processor; (e) filtering each small block of the discrepant block whose area is less than a second preset value from the discrepant block using the processor; (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 using the processor, 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 using the processor, or returning to block (a) if the total area is greater than or equal to the third preset value.