1. An endoscopic navigation method, comprising the steps of: receiving an image from a photographic device of an endoscopic navigation system in a lumen; performing image classification to determine whether the image is usable, wherein determining the image class comprises: when a ratio of pixels of the image with luminance values smaller than a first predetermined luminance value to the image size, is larger than a predetermined ratio value, the image is classified as a very dark image; when a ratio of pixels of the image with luminance values larger than a second predetermined luminance value to the image size, is larger than the predetermined ratio value, the image is classified as a very bright image; when a ratio of pixels of the image with hue values smaller than a first predetermined hue value or larger than a second predetermined hue value and saturation values larger than a predetermined saturation value to the image size, is larger than the predetermined ratio value, the image is classified as a double-exposure image; and when the image is not classified as a very dark image, a very bright image or a double-exposure image, the image is classified as a clear image; performing a first image process on the image to filter out dark areas of the image to produce a first processed image; performing a first determination procedure to identify the dark areas of the first processed image; producing a first result image for indicating lumen direction; performing a second image process to filter out fold curves of the image to produce a second processed image when there is no dark area in the image; performing a second determination procedure to identify the fold curves of the second processed image; producing a second result image for indicating lumen direction according to the fold curves; outputting the first result image or the second result image to a display device in the endoscopic navigation system to assist a user in operating the photographic device in the lumen.