Abstract: The ionospheric random irregular variety induces the amplitude fluctuation of very low frequency(VLF) wave, which is acute in the multimode interference region especially near the points where VLF field strength is minimum. Base on the standard deviation of the ionospheric reflect height, the fluctuation of attenuation rate and phase velocity for each mode are estimated with the theory of VLF wave propagation, then an estimation formulation of standard deviation for total filed is deduced. This formulation may be used both in the multi-mode and uni-mode regions, the calculation result of the formulation coincides well with the experiment datum. The formulation of standard deviation of amplitude fluctuation benefits the study about dependability of communications using VLF wave.