Abstract A chief problem encountered in georesistivity observation, namely interference from electromagnetic noise in measuring process, is discussed in this paper. The application of a new set of electric observation system PS-100, in Sichuan and Yunnan provinces, southwest China, is reported. The observation data from the four newly established stations indicates that the precision of the hourly georesistivity value reached 0.2% for the first time, and also that the CDMA-based PS-100 system has the high power of anti-interference ability, i.e. the capability to eliminate the influence of EM noise in georesistivity observations. Prior to the Ms5.6 Ludian earthquake on August 10, 2004, Yunnan province, co-seismic effect was recorded for the first time at Mianning station, which is 200 km far from the epicenter. The focal mechanism of the Ms5.6 event shows that Mianning station is situated at the compressive part of the primary wave Pn, which is consistent with the co-seismic georesistivity change. A change in georesistivity similar to that of gravity tide of M2 was observed at Mianning station, which can be interpreted as the of the stiffness tending to zero for the media at its surrounding area, and this may provide an example for the ‘model of impending earthquake precursor of geoelectricity triggered by tidal forces’.