The potential change of drought measured by the Palmer Drought Severity Index (PDSI) is projected by using a coupled climate system model under a Representative Pathway 8.5 (RCP8.5) scenario. The PDSI changes calculated by two potential evapotranspiration algorithms are compared. The algorithm of Thornthwaite equation overestimates the impact of surface temperature on evaporation and leads to an unrealistic increasing of drought frequency. The PM algorithm based on the Penman-Monteith equation is physically reasonably and necessary for climate change projections. The Flexible Global Ocean-Atmosphere-Land System model, Spectral Version 2 (FGOALS-s2) projects an increasing trend of drought during 2051-2100 in tropical and subtropical areas of North and South America, North Africa, South Europe, Southeast Asia, and the Australian continent. Both the moderate drought (PDSI < -2) and extreme drought (PDSI < -4) areas show statistically significant increasing trends under an RCP8.5 scenario. The uncertainty in the model projection is also discussed.