To meet the requirement of large-field, low-distortion and small-volume for optical system used in remote sensing field, a large-field freeform four-mirror off-axis reflective imaging system is designed with the 3D construction method, which is with the field of view of 60° and the relative aperture of 1/14. In this paper, the 3D construction method is introduced. A freeform optical system can be obtained directly from a starting point of conic system with the 3D construction method, and it can be taken as a good starting point for further optimization. In the final system, the average of the modulation transform function (MTF) of all field of view is greater than 0.5(25 lp/mm) and the maximum relative distortion is 0.54% . The 3D construction method is effective and can tremendously raise the design efficiency of freeform optical system. And the freeform largefield imaging system designed with 3D construction method shows its advantage of much lower distortion and meets the balance between the high imaging quality and low distortion of the large- field imaging system, satisfying the future development requirement of remote sensing.