Abstract: According to the crime of placarding notice and altering keyboard, cash exit, card entrance, combined with image preprocessing, contour tracing and different thresholds are used in subareas. Improved Laplace operator is used to extract the contour of ATM monitor image. The contour of ATM monitor image and template, consecutive contours are compared. When the difference of consecutive contour is more than given value and the same contour with template is less than given value, an alarm can be sent to monitor center in time. The practical ATM monitor system applies this technology. It is convenient to the watch. It improves the security and reliability of service.