This paper deals with the theory and application of the pre-processing method in the harmonic analisis of earth tides. Before using the Venedikov filters, we applied two step error detection to observation data. Forst, the data was smoothed by means of several filters. Then the occidental fault data, jump data or bad reading among the observation data were corrected, and thus the reading errors of observation, data were not more than 1 mm. Secondly, a rough least square adjustment was made on each 48 hour interval, that would be covered by a Venedikov filter {Nakai, 1975}. Then the bad groups of 48 hour interval could be detected out and a smoothing scale for each group could be given. By adopting this method the bad groups caused by anomalous drift in a short time (for example, 3-5 hours) might be corrected. Finally, the rest of bad groups, should be discarded in the computation of final results. Through the two step error detection, the accuracy of the computational results was promoted.