Using these simulations of NCAR CCSM3.0 climate model driven by natural and anthropogenic forcings (greenhouse gases,sulfate aerosol,ozone,black carbon aerosol,volcanoes and solar activity) in the project20th Century Climate in Coupled Models(20C3M) to firstly evaluate this models performances in the global monsoon region,solstitial mode of global precipitation, and the trend of global monsoon rainfall from 1979 to 1999,and then to discuss the possible effects of external forcings in the 20th century on the global monsoon rainfall trend. Results show that the global monsoon rainfall in full forcing simulation exhibits a distinct increasing trending the 20th century,which is mainly due to the change of the anthropogenic forcing, especially the greenhouse gases. The simulation driven by the greenhouse gases could produce Eastern Pacific(EP) cooling and the west pacific (WP) warming pattern,which enhances east-west thermal contrast in the Pacific Ocean and results in a rising pressure in the EP and a decreasing pressure over the Indo-Pacific warm pool. This enhanced Pacific zonal thermal contrast tends to amplify the water vapor merged into the monsoon region in the eastern hemisphere. The increasing landocean and inter-hemispheric thermal gradients will enhance the monsoon low pressure along with the water vapor convergence and the cross-equatorial flow,which will increase the global monsoon rainfall with a significant rise trend in the 20th Century. The global monsoon rainfall in the natural-only forcing experiment shows an increasing trend,but it is not significant at the level of 0.05. Sulfate and black carbon aerosols will produce the decreasing trends in the global monsoon rainfall trend in the 20th century,with this trend mainly in the northern hemisphere by the sulfate aerosol and in the southern hemisphere by the black carbon aerosol.