In order to get the compressibility factorZof working fluid under different conditions, experimental measurement method ofZunder high pressure and high temperature and data mining method were studied in this paper. Experimental measurement method based on real gas state equation and prediction method based on Least Squares Support Vector Machine were proposed. First, an experimental method for measuringZat high temperature and high pressure was designed; in this method the temperature, pressure, and density (mass and volume) of corresponding state were measured and substituted into the actual gas equation of state, and thenZcan be calculated. Meanwhile, in order to obtain continuous value inT-pplane, Squares Support Vector Machines are introduced to establish the prediction model ofZ. Take Hexamethyldisiloxane, for example; the experimental data ofZwas obtained using the experimental method. Meanwhile the prediction model ofZ, which can be used as calculation function ofZ, was established based on those experimental data, and theZ(T: 500 K~800 K,p: 1.3 MPa~2.25 MPa) was calculated by using this calculation function. By comparison with this published data, it was found that the average relative error was 2.14%.