According to the in situ measured data on 32 sampling sites in Chaohu Lake in June 2009, underwater light distribution was explored, as well as, the euphotic depth was calculated in these waters. Furthermore, primary productivity was evaluated with chlorophyll-a concentration. The results indicate that: the underwater light intensity power decayed with the depth, and at the depth of 1 m it decreased to only 0.07%-15.17% compared with that at water surface. In general, underwater light intensity was higher in the west and northeast lake at different water depth, on the contrary, it presented lower in northwest and east-central waters. At the same time, the euphotic depths ranged from 0.64 m to 2.33 m (mean 1.2 m) in Chaohu Lake. The maximum value was presented in the southwest lake; however, the euphotic depth was relatively lower in the northwest and east-central parts. Primary production values which were calculated by VGPM model were divided into three levels from 1363.77 mg/(m2 x d) to 7349.37 mg/m2 x d). Among these, Level 1 [0-2500 x mg/(m2 x d)] was mainly distributed in eastern waters and scattered in western region, and the total area was 312.66 km2; Level 2 [2500-5000 mg/(m2 x d)] located in the west and northeast and occupied 47.12% of the area (353.42 km2); simultaneously, Level3 [5000-7500 mg/(m2 x d)] with area of 83.92 km2 was distributed in the northwest and a small area in the central north.