GEO SAR can achieve better revisit time and observation performances when using high squint mode. However, high squint mode also causes huge range walk and severe range-azimuth coupling problems as well as severe 2-D spatial variance of the focusing parameters and range cell migration (RCM), which restrict the available scene size and increase the difficulty of GEO SAR imaging. To achieve high squint mode GEO SAR imaging, the effects of squint mode on imaging processing-especially the azimuth-variant RCM-are first analyzed in detail, and the relationship between the azimuth-variant RCM and the azimuth-variant focus parameters is discussed. Then, in this paper, a 2-D nonlinear chirp scaling algorithm (2-D NCSA) based on optimal azimuth polynomial compensation is proposed to realize high squint GEO SAR imaging. In the proposed algorithm, the optimal azimuth polynomial compensation which consists of a time-domain compensation and a frequency compensation removes the range walk, weakens the azimuth-variant RCM, and simplifies the imaging processing, while the 2-D NCSA which consists of a range NCSA and a higher order azimuth NCSA solves the 2-D spatial variance of focusing parameters and achieves high squint GEO SAR imaging. Computer simulations for different orbital positions validate the effectiveness of the proposed algorithm under high squint conditions.