For accomplishing the main mirror alignment of large aperture optic-infrared system,the small aperture sampling is introduced. Low order aberration of the main mirror is analyzed,which is helpful to system alignment. With the increase of optic-infrared system’s aperture,the traditional method can′t meet the requirement of the modern large system. Firstly,the basic principle of the small aperture sampling is introduced,then the hardware of the testing is presented for accomplishing this method,lastly,the validity and feasibility of the method are verified through numerical simulation.