1. An image processing method comprising steps of: obtaining a first image captured by a camera unit under a first field of view; generating a first rectangular mask in the first image, wherein the first rectangular mask has four end points and four boundaries connecting the four end points; converting first X-Y plane coordinates of the four end points in the first image into spherical coordinates; obtaining a second image captured by the camera unit under a second field of view; converting the spherical coordinates into second X-Y plane coordinates corresponding to the second image; determining whether the second X-Y plane coordinates of the four end points all are located in the second image; and if the second X-Y plane coordinates of the four end points all are located in the second image, generating a second rectangular mask in the second image by a maximum X coordinate value, a maximum Y coordinate value, a minimum X coordinate value and a minimum Y coordinate value of the second X-Y plane coordinates of the four end points.