By using the improved simultaneous physical retrieval method and with a 6 h numerical weather prediction as background field, the atmospheric parameters were retrieved from TIROS-N Operational Vertical Sounder (TOVS) data measured in January 1992 by satellite NOAA-12, and the water vapour retrieval errors were analysed and tested with conventional observational data as normal. The statistical results of the water vapour retrieval errors according to 3 satellite trajectories in the experimental area show some interesting features. The mean errors of retrieval for RH are closely correlated with the error of background field, and they are all negative. The regularity is shown obviously at high levels and high latitudes, that is, the absolute values of mean errors increase with altitude of level and latitude increasing. The interdiurnal variations of the errors are smaller at high levels, and bigger at low levels. The results of retrieval error corrected show that the retrievals above 500 hPa are better than those of background field, especially for trajectory A.