Leaf area index (LAI) is an important parameter of canopy structure, because it relates to many biophysical and physiological processes of canopy, including photosynthesis, respiration, transpiration, carbon cycling, precipitation interception, and energy exchange, etc. This paper introduced the theoretical bases and mathematical models of optical methods for forest canopy LAI determination, introduced the principles, merits, and drawbacks of currently used optical methods, and summed up the main sources of the errors in LAI optical measurement, including clumping effect, non-photosynthesis components, measurement conditions, and terrain effect. The developing status of quantitatively evaluating clumping effect, non-photosynthesis components, and terrain effect was analyzed, and the promising development directions of optical methods for measuring forest canopy LAI were discussed.