Organic Rankine cycle (ORC) has been demonstrated to be an effective and promising technology to recover low-grade heat source. Constant turbine efficiency is generally assumed in most of ORC studies, which is not necessarily accurate. To address this issue, a one-dimensional analysis model of radial-inflow turbine is presented. A comparative analysis of the thermodynamic and economic performances of ORC system with constant turbine efficiency and dynamic turbine efficiency is conducted for eight working fluids. The multi-objective grey wolf optimizer (MOGWO) is employed to conduct the multi-objective optimization of the ORC system with constant turbine efficiency and dynamic turbine efficiency. The optimization results are analyzed and compared to investigate the effects of the turbine efficiency on the working fluid selection and system parameters determination. A comparative analysis of the off-design performance of the ORC system with both types of turbine efficiency is also carried out, and the influence of the heat source inlet temperature on the multi-objective optimization results of the ORC system is also studied. The results show that the turbine efficiency increases with the decrement of evaporation temperature or the increment of condensation temperature, and there are significant differences of turbine efficiency among different working fluids. The optimal working fluid and operating parameters are different between the ORC system with constant and dynamic turbine efficiency. Isopentane and pentane are considered to be the optimal working fluids for the constant turbine efficiency ORC system, while only pentane is considered to be the optimal working fluid for the dynamic turbine efficiency ORC system. As the heat source inlet temperature increases, the difference in the optimal evaporation temperature and the net power output between the two ORC systems increases and the error caused by adopting constant turbine efficiency increase accordingly.