A measuring method to terrain atmospheric visibility with forward scattered light and transmitted light was introduced.Visibility is expressed by ratio of the intensity of the scattered light to the intensity of transmitted light.Spatial filters are utilized to limit visual angle of scattered light and transmitted light,and background interferences of measured forward scattered light and atmospheric scattered light.Using the correlation detection,atmospheric background noise is eliminated and signal-to-noise ratio for scattered light to background is enhanced.In order to keep spot of being measured light beam on the photo-detector unchanged,step index optical fiber is used in the set for improving the influence of thermal deformation on measuring error.Another characteristic is that changing in the light source and pollution to the optical elements may be ignored.The correlation coefficient between visibility from proposed method and accurate transmission method is about 98.2%.