1. A method for evaluating skin tissue, comprising: obtaining a tomographic image of skin; performing a quantization process for quantizing brightness values of the tomographic image of skin into a plurality of brightness levels to generate a quantized image; performing a binarization process on the brightness value of each image point in the quantized image according to a first brightness threshold interval to generate a first filtered image, wherein in the quantized image, image points whose brightness values fall within the first brightness threshold interval are set as bright spots in the first filtered image, and image points whose brightness values fall outside the first brightness threshold interval are set as dark spots in the first filtered image; performing the binarization process on the brightness value of each image point in the quantized image according to a second brightness threshold interval to generate a second filtered image, wherein in the quantized image, image points whose brightness values fall within the second brightness threshold interval are set as bright spots in the second filtered image, and image points whose brightness values fall outside the second brightness threshold interval are set as dark spots in the second filtered image; obtaining a first estimated tissue boundary according to the distribution of the bright spots in the first filtered image; obtaining a second estimated tissue boundary according to the distribution of the bright spots in the second filtered image; and estimating a thickness of skin tissue according to a difference between the first and second estimated tissue boundaries.