To conduct this research, first, the data of monthly observations of synoptic and hydrometric precipitation from the National Meteorological Organization and the Ministry of Energy during the 30-year period (2006-2005) were obtained. To examine the prospect of future rainfall changes, the historical data of the period (1976-2005) and the simulated climate data of the period (2050-2021) using two models of CM3), (CSIRO-Mk3.6 from the series) Models (CMIP5) and according to 4 scenarios RCP2.6, RCP4.5, RCP6 and RCP8.5) that are available with a spatial resolution of 0.5 x 0.5 with the BCSD method have been used.Mean-based (MB) strategy has been used to correct the bias in the output of these models. The results of the AOGCM models showed that the CSIRO-Mk3.6 error coefficient was less than the GFDL-CM3 model for simulating precipitation in the case of Large Karun.The average future rainfall (2021-2050) in the whole basin compared to the average observed rainfall during the statistical period of 1976-2005 shows, in both models and scenarios in both basins in terms of amount and area of ​​precipitation is decreasing significantly.Heavy rains in the Greater Karun Basin have been concentrated in all scenarios and models east of the basin. The highest rainfall was in the central foothills. The lowest rainfall is in the southwest and southeast. The final results of the present study are expected to be 83-116 mm. Both models are expected to have the highest rainfall in the Greater Karun Basin, with two scenarios: rcp4.5 and rcp2.6.