1. A method for acquiring a retina structure from an optical coherence tomographic image, comprising: calculating a region of interest (ROI) image of a single retina optical coherence tomographic source image; accumulating the single retina optical coherence tomographic source image in a transverse direction thereof to obtain a longitudinal-gray level curve; acquiring a maximum pixel gray level and a secondary maximum pixel gray level from the longitudinal-gray level curve; determining a height, a top boundary and a bottom boundary of the ROI image, according to a distance from a longitudinal position of the maximum pixel gray level to a top of the retina optical coherence tomographic source image, or a distance from a longitudinal position of the secondary maximum pixel gray level to a bottom of the retina optical coherence tomographic source image, and a preset constant; intercepting the ROI image according to the height, the top boundary and the bottom boundary of the ROI image; performing a Gaussian filtering on the ROI image; calculating a first estimated boundary position of a first layer boundary and a second estimated boundary position of a second layer boundary after the Gaussian filtering, using a multi-resolution method; simplifying an active contour model, and refining the first layer boundary and the second layer boundary respectively using the simplified active contour model according to the first estimated boundary position and the second estimated boundary position, to obtain a first initial boundary position of the first layer boundary and a second initial boundary position of the second layer boundary; smoothing the first initial boundary position and the second initial boundary position using a Savitzky-Golay filtering method, and taking the smoothed first initial boundary position and the smoothed second initial boundary position as a segmentation position of the first layer boundary and a segmentation position of the second layer boundary, respectively; acquiring segmentation positions of rest layer boundaries in the ROI image according to the segmented position of the first layer boundary and the segmented position of the second layer boundary, to obtain segmentation positions of all the layer boundaries in the ROI image; wherein pixel-projecting the retina optical coherence tomographic source image of the first frame in a transverse direction thereof to obtain a longitudinal-gray level curve specifically can be achieved through the following projection formula: g ⁡ ( y j ) = 1 M ⁢ ∑ i = 0 M - 1 ⁢ f ⁡ ( x i , y j ) , where, f(x i ,y j ) is a gray level of a pixel (x i ,y j ), i and j are horizontal (transverse) and vertical (axial) positions of the image, respectively, and M is a width of the image; wherein the simplified active contour model only contains an image energy item which is: E image = ∑ j = k + 1 j = k + n ⁢ ⁢ f ⁡ ( x i , y j ) - ∑ j = k - 1 j = k - n ⁢ ⁢ f ⁡ ( x i , y j ) , where, E image is an image energy item, f(x i ,y j ) is a gray level of a pixel (x i ,y j ), i and j are positions of the pixel (x i ,y j ) in transverse direction x and longitudinal direction y, respectively, k is a current pixel position in a layer boundary, and n is a longitudinal neighborhood radius of the pixel position k.