1. A method for retrieving atmospheric aerosol based on statistical segmentation, comprising steps of: S1: acquiring a multi-band remote sensing image of an analyzed area, and performing radiometric calibration to obtain a multi-band remote sensing image comprising an apparent reflectance, wherein the multi-band remote sensing image comprises a mid-infrared 1.6 micrometer band and a mid-infrared 2.1 micrometer band, and one band is selected from the multi-band remote sensing image as an retrieval band; S2: inputting the multi-band remote sensing image into an atmospheric radiative transfer model, obtaining a corresponding aerosol optical thickness look-up table according to the retrieval band, wherein the look-up table comprises parameters corresponding to each aerosol thickness value AOT for calculating the apparent reflectance; S3: for all pixels in the multi-band remote sensing image, performing segmentation using a statistical segmentation method, wherein the segmentation method comprises steps of: S3.1: partitioning an apparent reflectance range of the mid-infrared 2.1 micrometer band into N segments φ n according to a predetermined interval λ 1 , n=1, 2,. .. , N; S3.2: based on apparent reflectances of respective pixels of the multi-band remote sensing image in the mid-infrared 2.1 micrometer band, allocating the respective pixels into corresponding segments, to obtain pixel sets x n corresponding to respective segments φ n ; S3.3: for each pixel set x n obtained in step S3.2, in condition that a number of pixels in the pixel set x n |x n |≧T 1 , T 1 being a predetermined threshold value, retaining the pixel set, otherwise deleting the pixel set; marking retained pixel sets as x n′ , n′=1, 2,. .. , N′, N′ being a number of retained pixel sets; S3.4: partitioning an apparent reflectance range of the mid-infrared 1.6 micrometer band into M segments γ m according to a predetermined interval λ 2 , M=1, 2,. .. , M; S3.5: based on apparent reflectances of pixels in each pixel set x n′ at the mid-infrared 1.6 micrometer band, allocating respective pixels to a corresponding segment, to obtain pixel sets y n′m corresponding to respective segments γ m in the pixel sets; S3.6: in condition that a number of pixels in the pixel set y n′m |y n′m |≧T 2 , T 2 being a predetermined threshold value, retaining the pixel set y n′m , otherwise deleting it; marking retained pixel sets as y k , k=1, 2,. .. , K, K being a number of the retained pixel sets; S3.7: categorizing the K pixel sets y k retained in step S 306 according to a predetermined threshold T 3 , and in condition that a number of pixels in the pixel set y k |y k |≧T 3 , T 3 being a predetermined threshold value, categorizing the pixel set y k into a set Y s , otherwise categorizing it into a set Y t ; S4: marking a p-th pixel set in the set Y s as y s,p , p=1, 2,. .. , |Y s |, |Y s | being a number of pixel sets in the set Y s , performing retrieval on each pixel set y s,p one by one according to steps of: S4.1: partitioning an apparent reflectance range of the retrieved band into D segments ω d according to a predetermined interval λ 3 , d=1, 2,. .. , D; allocating respective pixels of the pixel set y s,p into a corresponding segment according to an apparent reflectance of the respective pixels at the retrieved band, to obtain a pixel set z s,p,d corresponding to each segment ω d in the pixel set; S4.2: obtaining pixel sets with a number of pixels |z s,p,d |≧T 4 by searching the D pixel sets z s,p,d , T 4 being a predetermined threshold value, selecting from the obtained pixel sets a pixel set d* having a smallest segment serial number d as a clean segment in the pixel set y s,p ; S4.3: designating an apparent reflectance of the clean segment z s,p,d* at the retrieved band as ρ* s =λ 3 ×d*, and designating a corresponding aerosol thickness value AOT s,p,d* =AOT 0 , AOT 0 being a predetermined aerosol thickness value of the clean segment; obtaining parameters corresponding to AOT s,p,d* from the aerosol optical thickness look-up table, and calculating the ground surface reflectivity corresponding to the clean segment z s,p,d* according to the apparent reflectance ρ* s ; S4.4: taking the ground surface reflectivity corresponding to the clean segment z s,p,d* as the ground surface reflectivity of the whole pixel set y s,p , and obtaining the aerosol thickness value of each pixel in the pixel set y s,p through retrieval; S5: performing retrieval on the set Y t according to the steps of: S5.1: traversing all pixels in the set Y s , to search for pixels with no values within a predetermined radius, designating an aerosol thickness value of a pixel with no value as the aerosol thickness value of the pixel, and marking all designated pixel sets as P s ; S5.2: for each pixel in the set P s , obtaining parameters corresponding to the designated aerosol thickness value from the aerosol optical thickness look-up table, and then calculating and obtaining the ground surface reflectivity of the pixel according to the apparent reflectance; S5.3: gridding the multi-band remote sensing image according to a predetermined side length; S5.4: marking a q-th pixel set in the set Y t as y t,q , q=1, 2,. .. , |Y t |, |Y t | being a number of the pixel sets in the set Y t ; for each pixel set y t,q , finding an intersection with P s from each grid partitioned in step S5.3, and making the ground surface reflectivity of all pixels of the pixel set y t,q in a current grid equal to an average ground surface reflectivity value of all pixels in the intersection; S5.5: performing aerosol thickness value retrieval on each pixel assigned with the ground surface reflectivity in the step S5.4, to obtain the aerosol thickness value of the pixel; S6: for remaining pixels with no values in the multi-band remote sensing image, designating aerosol thickness values thereof through interpolation.