An accurate knowledge of the atmospheric refractivity is important for the study of light wave propagation, and the refractive index of water vapor plays an important role in the atmospheric refractivity. The line-transitions of water vapor in the latest HITRAN (high resolution transmission) database are used to calculate the refractive index of standard water vapor for wavelengths between 0.3 μm and 20 μm. Before the calculation, a pseudo-spectral is constructed from the measured data to represent the UV resonances and photo-ionization continuum with nonlinear least square method, since the HITRAN database does not provide a line-transition in UV bands. Calculation results show that strong absorbance by water vapor occurs at wavelengths around 2.9 μm and 7 μm in the mid-infrared. Then a simple dispersive formula at wavelengths from 0.3 μm to 20 μm for standard water vapor is given, where the Gross line shape is used. Finally the new dispersive formula is compared with the previous formulae against measurements of refractive index for wavelengths in visible bands and around 3.4 μm, 10.6 μm, which is in good agreement with the measuremental data.