The geothermal organic Rankine cycle (ORC) power system may frequently operate under off-design conditions due to the variations of geological condition, ambient temperature and actual operation. To comprehensively investigate the off-design performance of ORC using zeotropic mixtures and the effectiveness of composition regulation system on off-design operation. Off-design performances of basic ORC using pure working fluid (BORC), ORC using zeotropic mixtures (MORC) and composition-adjustable ORC (CAORC) are compared. The off-design performance prediction model for each ORC is built based on specifically designed critical components. An optimal control strategy is adopted to maximum the net power output with various geothermal water mass flow rates, geothermal water inlet temperatures and cooling water inlet temperatures. The results imply that variations of control variables and operating parameters are similar for the three ORCs during off-design operation. Compared with BORC, the net power output of MORC and CAORC can be increased by 2.4-5.3%, and the increment is larger when the system efficiency of BORC is lower. Simultaneously, except under extreme conditions where geothermal water inlet temperature is very high and cooling water inlet temperature is quite low, the net power output of CAORC is only a little larger than that of MORC. (C) 2019 Elsevier Ltd. All rights reserved.