A new type of oscillation cavity is developed based on the optical characteristics of the combination of two rectangular prisms. The oscillation cavity consists of four rectangular prisms. The length of basic line and the number of oscillations were changed by adjusting the distances and locations of four rectangular prisms. So the optical length of the cell increased. The light intensity can be measured by photoelectric conversion, so a ratio and the new equation of calculating atmospheric visibility can be obtained. Because of the divergence angle of lasers, the diameter of the beam grows when the optical length increases. The situation is given by numerical simulation. The extinction coefficients corresponding with the different atmospheric visibilities are analyzed. The light intensities are calculated when the light passes the different optical lengths. This can provide reference for the selection of a detector.