Abstract: Design of underground heat exchanger is crucial for the application of ground source heat pump system.Hourly simulating is the most scientific method to design the underground heat exchanger.Xihua Geothermal Software for hourly simulating underground heat exchanger with vertical U-pipe is one self-developed.This software includes preprocess module, simulating engine and postprocess module.The program can hourly compute the inlet and outlet temperature of U-pipe and the ground temperature using hourly building loads as input data.Outlet temperature is compared with the field test.Resultsobtained from the comparison show that the program is correct.