Abstract: The calibration function for surface wave magnitude has been deduced theoretically from surface wave propagation in a layered medium, considering the effect of the linear filter composed of medium absorption, layered earth medium and response of the 763 long period seismograph on the amplitude and period of surface waves: 763(△) =3.9 + 0.16△/Tp + 1/31g△ + l/21g(sin△) + lgTp(1p is period corresponding to Airy phase (i.e. maximum amplitude of surface wave). This calibrating function is consistent with IASPEI(△) recommended by IASPEI at epicentral distance range 20-160 and able to fit quite well with observed data in the broader range 2-178. Extension of epicentral and period ranges for IASPEI(△) has been effected. This case is important for China because most earthquakes occurring in China and recorded by Chinese seismic network are at epicentral distances of 1 -20.The surface wave magnitudes Ms,763 determined in terms of 763(△) for 103 earthquakes recorded by 763 long period seismic network in China have been calculated. The standard deviation is 0.14 magnitude unit, less than that (0.20) measured by median period broadband seismograph (SK) network. In comparison with Msz measured by NEIS using WWSSN, it is shown that there is no systematic difference between Ms,763 and Msz statistically, i.e. the average value of △Ms = Ms,763-Msz, is close to zero (0.009) The standard deviation of △Ms is 0.21.