1. A method for detecting activity within a fixed view by using a monitor system, the monitor system comprising a camera for filming the fixed view at a predetermined camera speed and a calculator for detecting the activity within the fixed view according to a first image and a second image generated by the camera, the second image being generated within a predetermined time interval after the first image is generated, each of the images generated by the camera comprising a plurality of blocks, each of the blocks comprising a plurality of pixels, the method comprising: calculating an average value A for each block of the second image, each of the average values A is equal to an average of differences between pixels of one of the blocks of the second image and pixels of a corresponding block of the first image; calculating a mean difference B between an average value of pixels of the first image and an average value of pixels of the second image; calculating a difference V between the average value A and the mean difference B for each block of the second image, and selecting blocks of the second image that have corresponding differences V greater than a threshold value; and determining whether there is any object moving within the fixed view according to the selected blocks.