The geothermal energy is renewable energy, which is viewed as a promising alternative to fossil energy. This paper conducts a comparative study among double-flash, flash-Kalina and flash-ORC geothermal power plants. The mathematical models of three different geothermal power plants are established. And parametric analyses are performed to examine the effect of some key parameters on system performance. The performance of each cycle has been discussed in terms of the net power output, thermal efficiency and exergy efficiency. Moreover, genetic algorithm (GA) is used to conduct optimization to obtain the best performance of three different geothermal power plants. Flash-ORC geothermal power plant yields the highest exergy efficiency of 46.121%. Exergy efficiency of Flash-Kalina geothermal power plant is 36.747%, but the inlet pressure of ammonia-water turbine is much higher than that of organic turbine and steam turbine. Double-flash has the lowest exergy efficiency of 31.922%.