The invention discloses an effective image area detection method and an effective image area detection system. By the method and the system, the aims of automatically identifying the effective area of an analogue signal and regulating an optimal display position are fulfilled, and the problem of blackedging in the display of the analogue image signal is effectively solved. The method comprises the following steps of: detecting whether image data corresponding to each pixel clock belongs to an effective image or not, and recording coordinate values of the image data; taking counted minimum rowand column coordinates as starting coordinates of the effective image area, and taking counted maximum row and column coordinates as the ending coordinates of the effective image area; and finally making a range judgment on a detection result, and outputting an effective signal if a range is legal.