The article deals with building a seasonal model with a linear growth of the Winter’s statistic value, on the basis of which the parameters are calculated: the customs value index and the weighted average rate, which are the composition of the calculation formula of customs payments. To build the model, the monthly data of the customs statistics on imports of 28 commodity group “Products of inorganic chemistry; inorganic or organic compounds of precious metals, rare earth metals, radioactive elements or isotopes” of the Customs Nomenclature of the external economic activity of EAEU from 01.01. 2018 up to 31.12.2020 are used in the region of activity of the Southern Customs Administration.

Keywords: time series, econometric modeling, forecasting, adaptation parameter, seasonality coefficients, adaptive model