A computing system displays current real-time images of an area monitored by a camera. The computing system includes a motion detection unit. The motion detection unit determines a number of varied pixels in the real-time image compared with a previous image, and to determine a ratio of pixels of the number of the varied pixels to a total number of pixels in the real-time image. If the ratio is greater than a predefined number, the computing system increments an abnormal pixel count by one. If the abnormal pixel count is greater than a maximum abnormal pixel number, the computing system starts an alarm device connected to the computing system.