Abstract: We retrieved Rayleigh wave Green functions between all possible station pairs after pre-processing, cross-correlation and stacking the data, which are 21 months' continuous ambient seismic noise recorded by 100 evenly distributed broadband seismic stations mainly in the National Seismic Network in central and eastern Chinese mainland. The quality of the empirical Green functions (EGFs) and the dispersion curves along selected paths were estimated to measure the reliability and stability of the EGFs. Our results display that the EGFs are correlated with the earthquake signals, suggesting the reliability of the EGFs. In addition, we counted the number of dispersion measurements with SNR>10 retrieved from the time-series of different lengths (3—21 months). The result shows that, to obtain enough reliable EGFs for velocity tomography, at least 12 months' data should be used and 12 months’ data can make sure that dispersion curves under 30 s are temporally stable.