The invention discloses an automatic building identification method based on a Fourier descriptor. The method is used for automatically identifying illegal buildings of illegal land use. The method comprises the following steps: an original building RGB color image is converted into a gray edge image through filtering, enhancement and edge detection; all the contour of the gray edge image is extracted according to a Fourier descriptor method; the contour points are written in the plural form, discrete Fourier transform is carried out, and then modulus operation is performed on the value after transformation to obtain a normalized Fourier descriptor; the extracted contour data is simplified according to a DP algorithm; a building shape standard template library is established according to the simplified building contour; the Euclidean distance with the standard template library is calculated, and the Euclidean distance is used to represent the similarity of two Fourier descriptors; and finally, whether the extracted contour is a typical building shape feature is judged. Therefore, the purpose of automatically identifying an illegal building in a video image is achieved.