Abstract: Wavefront sensor-based beam cleanup adaptive optical system is the main equiment to improve the beam quality of high-energy laser systems. However, the system is complicated and bulky because it requires to measure wavefront and needs a strong beacon source. To solve the above problems, this paper proposes a wavefront sensorless adaptive optical system which utilizes the deformable-mirror eigen modes and the characteristics of the far-field spot for processing and analyzing thus to correct the square beam. The eigenmode decomposition of the deformable mirror’s influnce function is used, and the mean square radius of the far-field spot is used as the metric function. The relationship between the eigenmode coefficient of the wavefront and the metric function is established. The measurement of the metric function is used to calculate voltage. Simulation analysis and experimental verification of the correction method show that the method can effectively achieve the correction of static aberrations and improve the energy concentration of the far-field spot.