The article deals with development of new spectral method for determination of spectral indices used to estimate the soil moisture content. The main disadvantage of such indices is that they use spectral absorption lines of water vapor. Consequently, using them requires adequate compensation of the atmospheric influence. The solution of this issue was carried out on the basis of the following provisions: (a) it is assumed that a model expression of the reflection spectrum dependence on the moisture content in the soil is known; (b) there are experimentally removed curves of the specified dependence; (c) the function of the experimentally measured soil reflection value dependence on the degree of soil moisture is introduced; (d) the search for the optimal type of the introduced function is carried out at which the square of the difference between the experimentally measured value of reflection and the known model function reaches the minimum. In this case, the specified search is carried out by the variational optimization method, subject to the introduction of some restrictive condition on the desired function, using continuous discrete sums form recording; (e) equating the calculated optimal function with the experimentally obtained reflection value at a specific wavelength, the water content of the soil is calculated. At the same time, permanent using of wavelength with minimal water content is recommended. The described experimental researches confirmed effectivity of the suggested method.