Abstract: This paper presents the principle and measuring method of a laser instrument used to measure runway horizontal visibility and approach visibility. By means of the co operation target,the instrument could simply measure approach visibility,greatly increase amplitude of receiving signal and improve its sensitivity.PIN photodiode can be used as photoelectrical transformation unit to detect the amplitude of receiving signal. By means of transmission ruler,curves of relative errors are optimized. Measurement precision of visibility,especially lower visibility is improved.Measurement error of runway horizontal visibility and approach visibility is less than 12%.