The geothermal heat exchanger is the key component of groundcoupled heat pump systems. We discuss the key parameters method of calculating geothermal heat exchanger length based on linesource theory. Typical meteorological year data is used to determine the hottest month, the coldest month and annual average surface temperature. The building hourly load can be obtained by introducing the concept of equilibrium temperature. The cooling fraction and the heating fraction then can be calculated using building hourly load and performance parameters curvefit of a water source heat pump unit. The method for selecting the hottest and lowest entering fluid temperature and calculating the borehole and ground thermal resistance is given. A vertical Utube heat exchanger length calculation procedure is proposed.