The article discusses the compilation of an algorithm for calculating the water-energy mode of a hydroelectric power plant cascade from particular mathematical models, integrated into a single mathematical model to take into account the variability of external and internal factors, electrical and hydrological relations, and technological restrictions, imposed on the process of its functioning. Drawing up a capacious algorithm for an interconnected process between two cascade hydroelectric power plants is based on Boolean algebra and includes two main modules of particular mathematical models 'Electricity' and 'Water' in the calculation. Verification of the algorithm was carried out in Eurostag. The obtained results showed that the developed algorithm for calculating the water-energy mode of the HPP cascade has a negligible calculation error, relative to the mathematical calculation and allows to determine with high accuracy the main parameters of the hydro power plant cascade operation mode. It is necessary to provide the power system with the required power, taking into account not only the magnitude of the water flow through the reservoir, but also the mode of its operation over long periods of time. The algorithm takes into account the possibility of idle discharges, monitors and identifies the reference points of unacceptable deviations, correcting the calculation error at each step, necessary for it. This ensures an increase in the mode controllability of the process of generating electricity through the hydro power plant cascade. © 2023 IEEE.