1. An image segmentation threshold value deciding method, applied to an image sensor, for deciding a first image segmentation threshold value of the image sensor, comprising: (a) defining at least one image region of a first sensing image; (b) deciding a first part image segmentation threshold value according to a first image region from the image regions; (c) deciding a second part image segmentation threshold value according to a second image region from the image regions; (d) performing a first image segmentation operation to the first sensing image via the first part image segmentation threshold value to acquire a first segmented image; (e) performing a second image segmentation operation to the first sensing image via the second part image segmentation threshold value to acquire a second segmented image; (f) selecting one of the first part image segmentation threshold value and the second part image segmentation threshold value as the first image segmentation threshold value according to the first segmented image and the second segmented image.