Abstract: Deformation monitoring system is the safeguard of dam.Space estate and time characteristic of dam are attained by deformation monitoring,which is used to investigate the stability,discover the problems,take some measures,study deformation mechanism,validate designing theory as well as set up proper prediction model and methods of the dam.The real-time effective data processing and analysis are important in dam deformation monitoring.Dynamic deformation is taken as placidity random course.Dam deformation as well as water level change of reservoir and temperature variation is taken as linear system in this paper.Fast Fourier Transform transfers those signals from time domain to frequency domain. The deformation data,including water level of reservoir,are taken from the database of Geheyan GPS auto-monitoring system from June 10,1998 to October 23,1999,in total 491 days.Temperature data only about 343 days which are between June 10,1998 and May 18,1999.Those data are prepared as 4 data each day by tri-spline function.A difference filter is used to remove long-term trend.The Power spectrum of dam deformation,water level and temperature are shown in Fig.5,Fig.6 and Fig.7.From the figures we know their main frequency are near 1 Hz(about 1 day).Day change of water level of reservoir and temperature is one of main reasons of day deformation of dam.Delivering function curve of deformation and water level is shown in Fig.8.Delivering function curve of deformation and temperature is shown in Fig.9.The signals energies both reach max value at 1.884 Hz.Power spectrum of velocity of dam deformation,water level of reservoir and temperature are shown in Fig.10,Fig.11 and Fig.12. A discussion is carried out at the end of this paper.In order to verify the performance of the result derived from spectrum analysis,cross correlation between deformation and water level as well as temperature is studied,and the result is presented in Fig.13 and Fig.14.However,there is only one temperature data available,but four temperature data are required for analysis.From the discussion we knew that it is a big problem in such a condition.At last two conclusions are drawn:1) Day change of water level of reservoir and temperature is one of main reason of day deformation of dam.If there were only two reasons,the effect of water level is about 63% and temperature is about 37%.2) Power spectrum of velocity of dam deformation,water level of reservoir and temperature shown very similar and their energy is mostly in high frequency. The velocity of water level of reservoir and temperature is one of the reasons of velocity of dam deformation.But there are still some further work to do in future.Long-term data and more data for each day should be studied further if possible.