This paper introduced a visibility detecting system of double light path based on the theory of infrared foreword scattering, which has two transmitters and two receivers. The system includes three parts: emitters, detectors and signal processor. This article describes its theory, principal components meanwhile discusses, it solves the technical difficulties in designing circuitry for detection ... [Show full abstract] and amplification of the weak signal. It is explained that the precision and stabilization of the visibility meter were improved by pairs of optical path and lock-in amplifier in receiving side. And such improvements have been certified by experiments.