Abstract: The total precipitable water in atmosphere is one of the most important parameters in astronomical observation and meteorology. A new instrument for measuring the total precipitable water in the atmosphere is introduced. The near infrared spectra of sunlight or moonlight passing through atmosphere were measured by a CCD spectrometer in the instrument, then the central residual intensity of the water vapor absorption at 935nm waveband was calculated and the total precipitable water in the atmosphere was derived by R=0.59W model. The calibration method, performance, advantages and disadvantages of the water vapor measurement instrument were discussed and measurement results were given. The measurement results indicate that this method is straightforward and its performance is stable.