In this paper,the Nakai's fitting method is improved and it is applied to the calculation of tidal and barometric coefficients of the well level.The advantage of this method is that the lag time of the response of the well level to the tide and barometric pressure can be calculated.The well Lu-06 is instanced.We have calculated by this method that the tidal coefficient is 16.9mm/10^-8,the barometric coefficient is 5.58mm/hPa,their lag time respectively is 18.8 and 22.4 minutes for the well level.The corrected standard deviation decreases from 8.5mm to 6.1mm.