This study was designed to determine the effect of rainbow trout cage culture on the chlorophyll-a content and trophic index value in the Abbas Abad area, southern Caspian Sea. For this purpose, water samples were collected by a distances of 5, 50, 100 and 1000 m from the cages during December 2014 (before rearing period), March and April (during rearing) and August 2015 (post rearing period). In this study, a totalof 42 species of phytoplankton belonging to five phyla (with the dominance of Bacillariophyta) were identified. Chlorophyll-a content varied significantly in different periods and stations (P<0.05). The highest and lowest chlorophyll-a content was recorded in March and station 5 m (2.13 μg/l) and May and station 1000 m (0.44 μg / l) away from cages, respectively. The Trophy State Index (TSI) was significantly different between the sampling periods (P<0.05). The highest TSI (42.18) was observed in March and then in August (40.47) and this differences were significant between different periods (P<0.05), whereas, it was not affected by the distance from the cages (P<0.05). It seems that the rainbow trout cage culture in the Abbas Abad region no remarkable effect on Chlorophyll-a and trophic index in the vicinity of fish cages due to the small scale and short duration of cage culture activity and strong water currents, so that observed changes were more associated with seasonal fluctuant.