A direct construction method for freeform surface optical system is proposed to address the problem that the initial structure is difficult to obtain in the design of such system. With the ideal object-image relationship and the Fermat principle as the criteria of single module iteration, an initial structure with favorable optimization potential can be directly obtained from a plane optical system without optical power by tracing light rays point by point. It can be solved faster by sampling feature light rays with different densities at different iteration stages. After the initial optical structure is obtained, optical design software can be used to further optimize the image quality. A compact off-axis two-mirror optical system with a 150 mm focal length and a physical size less than 40 mm×70 mm×60 mm is designed by the proposed method, which verifies the feasibility of this method in the design of freeform surface optical systems.