The active laser underwater imaging system of far distance puts strict demands on the expanding ability and adaptability to the distance of the expanding system.An expanding optical system is studied and designed for getting appropriate illuminating area at different imaging distance.The expanding system with the special rule that laser beams apply when transferring and transforming is analyzed.The mathematic relation between target position and modulating of the expanding optical system is discussed.A controlling project of the expanding optical system in laser underwater imaging system is raised at last.