The invention relates to a method and a system for detecting a star-moon pattern in a video image. The method comprises the steps of setting a region of interest in a frame of video image; performing foreground detection on the region of interest according to a preset foreground detection method to obtain a foreground image; detecting the star-moon pattern by using a classifier with preset star-moon characteristics in a contour of the foreground image; marking the foreground image and performing outward expansion by taking a region where the foreground image is located as a center to form a preset expanded region as a statistic region of detection of subsequent frames of images; detecting the star-moon pattern in the statistic region of a predetermined number of continuous frames of images after the frame of the foreground image by using the classifier; and outputting an alarm prompt when a detected frequency meets a condition. Therefore, an effective detection means is provided for effectively performing star-moon pattern detection in the video image.